I. BREXIT AS CONSTITUTIONAL EVENT

As a constitutional event, Brexit is unprecedented in both the EU and UK's constitutional arrangements. From the UK's perspective, few events have had such a profound impact on the UK's legal infrastructure as the UK's accession to the EU's sophisticated legal order and the gradual implementation of the latter into the former over the decades by the UK courts. From the EU's perspective, no Member State—let alone an economically and militarily powerful one—has ever left the bloc, thus posing existential questions about the future security and viability of the integration project.

Here one discrete aspect of the Brexit process will be analysed: namely the extent to which it challenges constitutional pluralism as both an analytical and normative account of the interactions between the EU and UK's legal and political orders. Initiated in the work of Neil MacCormick,Footnote ¹ constitutional pluralism involves the idea that two autonomous legal and political systems can interact at a high degree of intensity, making simultaneous claims to ultimate authority, without one being subordinated to the other (descriptive constitutional pluralism);Footnote ² as well as the idea that conflicts between two systems interacting in this way should be resolved according to prudential judicial politics,Footnote ³ or principles, shared by,Footnote ⁴ or external to,Footnote ⁵ both systems (normative constitutional pluralism).Footnote ⁶ Key to the idea of constitutional pluralism in both its forms is that the systems are organised in a heterarchial rather than a hierarchical arrangement.Footnote ⁷ As such, a key premise of constitutional pluralism is that the hierarchal concept of sovereignty is either no longer relevant to such relationships,Footnote ⁸ or is in transition from the ‘high sovereignty’Footnote ⁹ of the Westphalian world view of independent sovereign states to a model of ‘late sovereignty’.Footnote ¹⁰

To the extent that the vote for Brexit was about ‘taking back control’, it seems to suggest a ‘rewind’Footnote ¹¹ to a classically Westphalian worldview, where states are the exclusive authorities over their territories, and alien norms, including those of supranational entities such as the EU, are valid only through the recognition of, and to the extent permitted by, the state legal order in the exercise of their (Westphalian) sovereign powers. From a legal perspective, then, Brexit seems to suggest a move from a novel ‘post-sovereign’ arrangement predicted on constitutional pluralism to a classic dualist or Westphalian understanding of the relationship between the UK and the EU. Moreover, the potential impact of Brexit on the European integration project has prompted calls for an end to the ambiguities of ‘post-sovereign’ models of organisation and a shift to a more unambiguous understanding of the relationship between the EU and its Member States by forging a clearer path to (hierarchical) EU federalism.Footnote ¹² In these ways, Brexit seems to question constitutional pluralism as a way of explaining the interactions between EU law and UK law and as a guiding political principle of European integration. This article challenges both of these views.

The article proceeds as follows. Part II gives a brief account of how EU law has sedimented into the UK constitution in the years since the UK joined the organisation in 1973. In the absence of explicit statutory recognition of the primacy doctrine in the European Communities Act 1972, it provides an overview of the necessarily judicial negotiation of the balance between primacy and parliamentary sovereignty. Part III makes the case that this judicial negotiation of the status and effect of EU law is best captured by constitutional pluralism as a model of interacting legal orders, when compared to alternatives such as monism and dualism. Part IV goes on to consider reforms in this relationship post-Brexit through an analysis of the key provisions of the European Union (Withdrawal) Act 2018 (‘EUWA’), concluding that we can expect a relationship of constitutional pluralism to continue well into the future after the UK exits the EU based on the provisions of this Act. Part V turns to the EU level to consider the role that constitutional pluralism can play in future constitutional reform in the EU as a response to its multiple crises. Emphasising changes in the post-Brexit political landscape among the remaining EU 27, particularly in terms of enhanced support for the EU and the solidarity between the EU 27 during the Brexit process, it concludes that constitutional pluralism should continue to be a guiding principle of EU constitutional reform into the future.

II. A NEGOTIATED UNSETTLEMENT: EU LAW IN THE UK CONSTITUTION

In order to assess the relevance of constitutional pluralism post-Brexit, it is necessary to revisit the story of the assimilation of EU law into the UK constitution prior to the UK's eventual withdrawal. As is well known, the European Communities Act 1972 (‘ECA’) created the structural links between the UK constitution and the EU's legal order, and between the UK courts and EU courts. In particular, it gives expression to the unique features of the EU legal order; the fact that it is a ‘constitutional’ system that makes independent claims to validity and authority as expressed in the doctrines of direct effect and primacy.Footnote ¹³ Significantly, however, only one of these core features is expressly provided for in the Act itself. Section 2(1) of the Act, in providing that ‘[a]ll such rights, powers, liabilities, obligations and restrictions’ arising under the EU Treaties are ‘without further enactment’ to be given legal effect and ‘recognised and available in law’, clearly introduces the direct effect of EU law into the UK's legal system.Footnote ¹⁴ That the primacy doctrine existed when the UK joined,Footnote ¹⁵ and was unambiguously incorporated in the UK's legal order, is clear from its status as an ‘obligation and restriction’ created ‘by or under the Treaties’ under Section 2 ECA. Moreover, as a judicially developed doctrine embedded in the case law of the EU courts, UK courts are bound to apply it under Section 3 ECA.Footnote ¹⁶ However, explicit mention of the doctrine itself is notably absent from the Act. As a consequence, the ECA is completely silent as to how conflicts between EU law and the UK legal order should be managed,Footnote ¹⁷ with the reconciliation of the primacy of EU law and parliamentary sovereignty left to the courts.

The first significant recognition and application of the principle of primacy in the UK constitution was in the celebrated Factortame decision.Footnote ¹⁸ However, notwithstanding its status as a major milestone in the accommodation of EU law into the UK constitutional order; the case, somewhat surprisingly, has little to say about the relationship between primacy and parliamentary sovereignty. Whereas the case confirmed that where an Act of Parliament and EU law conflicted, the Act should be set aside, it did little to clarify the relationship.Footnote ¹⁹ It does not stipulate the terms and conditions under which displacement should take place, whether there are limits to this statutory displacement, and, more generally, how EU law rearranges the UK's constitutional settlement based on the sovereignty of the Westminster Parliament.

The Thoburn case provided a little more clarification on this relationship by distinguishing between ‘constitutional statutes’ and ordinary statutes in the UK constitutional system.Footnote ²⁰ The Divisional Court found that—as a ‘constitutional statute’—the ECA is immune from implied repeal.Footnote ²¹ However, at the same time as finding that the ECA, and therefore EU law, had an authoritative status in the constitution in this way, the court made clear that this authority did not displace or eradicate the constitutional authority of parliamentary sovereignty,Footnote ²² and that where parliament wished to override a provision of EU law it could do so in express statutory terms.Footnote ²³

The HS2 decision provided further nuance to the relationship between the primacy doctrine and parliamentary sovereignty in the absence of any explicit direction by the ECA itself.Footnote ²⁴ The UK Supreme Court (‘UKSC’) found that where EU law required that one of the key manifestations of the doctrine of parliament sovereignty—the embargo on judicial scrutiny of legislative procedure—be suspended,Footnote ²⁵ it would not be able to undertake such an investigation and that the doctrine of the primacy of EU law may have to give way to the doctrine of the supremacy of the UK legislature.Footnote ²⁶

Whereas a balance between the doctrine of primacy and parliamentary sovereignty was not directly at issue in the Miller case,Footnote ²⁷ the Court found that the effect of EU law on the constitution was ‘unprecedented’,Footnote ²⁸ such that the ECA made EU law not only a source of law in the UK but a ‘direct source of law’Footnote ²⁹ which ‘takes precedence over all other sources of law, including statutes’.Footnote ³⁰ Furthermore, and somewhat surprisingly, the UKSC found that whereas the ECA gave effect to EU law, ‘it was not itself the originating source of that law’;Footnote ³¹ EU law constituted an ‘independent and overriding source of domestic law’.Footnote ³² However, all of the exceptionalism surrounding the ECA as a constitutional statute and EU law as an ‘independent and overriding source of domestic law’, was ultimately subject to the power of the UK Parliament to withdraw the UK from the EU and repeal the ECA. Thus, notwithstanding its exceptional nature, the role of EU law in the UK constitution was still, ultimately, subject to the ‘rule of recognition’ which identifies Parliament as the ultimate source of law under the constitution.Footnote ³³ Parliament could repeal the ECA and EU law would cease to be a source of law under the constitution. As such, EU law could only enjoy a status ‘in domestic law which the principle [of parliamentary sovereignty] allows’.Footnote ³⁴

The judicial negotiation of the tensions between the primacy doctrine and the doctrine of parliamentary sovereignty can be characterised in the following terms. The primacy doctrine is clearly available and applicable in the UK constitution. EU law is an important source of law in the UK constitution in that it owes its existence to law-making processes outwith the constitution and it enjoys an independent and original authority.Footnote ³⁵ However, notwithstanding the original authority of EU law, its recognition within the UK constitutional system is not without conditions. First of all, the recognition of primacy occurs by virtue of the UK constitutional system itself. As such, the primacy of EU law is ultimately subject to the maintenance in force of the ECA. EU law cannot prevent Parliament repealing the ECA should it so wish. Moreover, even if Parliament does not exercise its ultimate sovereignty to repeal the ECA, there are still limits to the extent to which courts will apply the primacy doctrine. Where important constitutional foundations are at stake, the UK courts may subject the primacy doctrine to those constitutional principles including parliamentary sovereignty.

III. REPRESENTING THE RELATIONSHIP BETWEEN EU LAW AND THE UK CONSTITUTION

Reviewing the key features of the case law of UK courts on the accommodation of EU law reveals an ambiguity as to the specific status of EU law in the UK constitution and the precise balance between the primacy doctrine and the doctrine of parliamentary sovereignty. There is no clear and simple ‘showdown’ where it can be definitively concluded that the doctrine of parliamentary sovereignty remains completely intact, nor the primacy doctrine given unconditional precedence. Whereas outer limits have been alluded to, such as in HS2, they have never been thoroughly explored nor precisely defined. The identification of the ECA as a ‘constitutional statute’ precludes simple readings of parliamentary sovereignty as being equated with the continuing sovereignty of all parliaments at least as far as the doctrine of implied repeal is concerned.Footnote ³⁶ Furthermore, the ambiguity surrounding the nature and impact of EU law on parliamentary sovereignty is clearly in evidence in the UKSC's ambiguous comments in the Miller decision whereby it was simultaneously an ‘independent and overriding’ source of law under the UK constitution, yet also derivative and dependent on the ECA.Footnote ³⁷ This nuanced and negotiated ‘unsettlement’Footnote ³⁸ of the status of EU law in the UK illustrates quite clearly, it is argued, the relevance of the analytical model of constitutional pluralism to the UK/EU relationship. This can be appreciated by comparing constitutional pluralism with the alternative extant models accounting for the relationship: monism and dualism.Footnote ³⁹

Monism is, perhaps, the least plausible model. On a monist reading, the relationship between EU law and UK constitutional law would be ordered hierarchically such that UK law would always be subject to EU law in cases of conflict and that UK law owed its validity, not to the doctrine of parliamentary sovereignty, but to the EU's legal order.Footnote ⁴⁰ This reading of the relationship between EU law and UK law has some affinity with the original ‘constitutionalising’ judgments of the Court of Justice of the European Union (‘CJEU’) and could support the direct effect and primacy of EU law.Footnote ⁴¹ However, whereas the CJEU claimed that the ‘special and original’ nature of EU law necessitated the doctrines of direct effect and primacy, it never claimed that national law owed its validity to EU law. Moreover, as has already been seen from the statements of the UK courts, there is no evidence that this is how the relationship between the two operate within the UK constitution.Footnote ⁴² The one clear and consistent theme from the key cases on the question is that UK courts deem EU law to be valid and effective in UK law by virtue of an Act of Parliament and not by virtue of the ‘special and original’ nature of EU law itself.Footnote ⁴³ Furthermore, as this relationship has developed, the courts have not accepted the CJEU's understanding of the primacy doctrine that in all cases that domestic law—including core tenets of the domestic constitution such as those at stake in HS2—will be compromised in the application of EU law. This rules out any simple monist reading of the relationship between EU law and the UK constitution.

Dualism involves the positing of two legal orders operating and interacting in particular ways, where one, or possibly both make claims to primacy or hierarchy in cases of conflicts between them.Footnote ⁴⁴ From the viewpoint of domestic courts, this has usually involved interactions between state law and international law, and dualism as a doctrine of domestic constitutional law usually posits the supremacy of the domestic constitution over conflicting provisions of international law.Footnote ⁴⁵ In the context of EU-UK relations, then, dualism would find the British courts accepting that EU law was a distinct legal order which was autonomous from the UK constitution. However, its distinctiveness exists only by virtue of the UK constitution and its statutory recognition in the ECA. Moreover, in cases of conflicts between the EU and UK constitutional orders, the latter would prevail in all circumstances.

In some ways, dualism seems to provide a good account of the accommodation of EU law in the UK constitution outlined above. For example, the idea endorsed by the UK courts that EU law is valid and has effects in UK law by virtue of the UK constitutional order itself and in particular the ECA, entails classically dualist overtones. However, beyond this, it is not clear that dualism offers a good account of the relationship. With regard to the authority of EU law, dualism would insist that its authority in the UK system is wholly attributable to that particular system. However, in Miller the UKSC recognised that EU law was an original ‘independent and overriding’ source of law within the UK constitution,Footnote ⁴⁶ something which dualism cannot account for. Moreover, with regard to the question of conflicts, dualism would insist that any conflicts always be resolved in favour of conventional UK constitutional law, and in particular the doctrine of parliamentary sovereignty as conventionally understood including the doctrine of implied repeal. However, as the cases considered above show, conflicts between EU law and UK constitutional law are more complex than the dualist position allows. The Factortame decision is a testament to the fact that conflicts will frequently, and arguably predominantly, be resolved in favour of EU law. Thus, while arguably an improvement on monism, it is submitted that dualism offers an unsatisfactory model to understand the relationship.

The deficiencies of dualism can be rectified by constitutional pluralism as an account of the interactions between EU and UK law. Like dualism, it posits two constitutional orders of equal standing, each making their own independent claims to constitutional authority, operating side-by-side and interacting in certain ways through a structural framework such as the ECA ‘portal’ in the UK.Footnote ⁴⁷ Moreover, unlike monism or dualism, constitutional pluralism explicitly rejects any all-purpose resolution of one system over the other; rather the two orders are mutually accommodating without directly challenging the authority of the other.Footnote ⁴⁸

In analysing the extent to which constitutional pluralism provides a good account of this relationship a number of features can be emphasised, beginning with the recognition of plural claims to constitutional authority. In the cases discussed above, the original constitutional authority of EU law was clearly recognised in a way that had practical consequences for the application of UK constitutional law. Furthermore, while these claims were recognised, the constitutional claims of the UK constitution were also recognised. Perhaps nowhere is this clearer than in the Miller case where the UKSC simultaneously upheld the original and independent nature of EU law as a source of UK constitutional law as well as the constitutional authority of the UK Parliament. Moreover, the definitive resolution of the question of ultimate authority in favour of one particular legal order is explicitly rejected by constitutional pluralism and is not, as noted above, a good account of the more nuanced accommodation of EU law within the UK constitution in the cases considered above. If Factortame appeared to provide an all-purpose resolution of EU law over the UK constitution in that EU law seemed to have subordinated the sovereignty of Parliament through the disapplication of an Act,Footnote ⁴⁹ then Thoburn and particularly HS2, provide a counter-point to this view based on the constitutional authority of the UK constitution. Indeed, in explicitly stating that there were limits to the extent of the application of EU law in the constitution as set by key fundamental principles, and offering insights into what those principles might be, the UKSC's HS2 decision seems to come directly from the playbook of the German Federal Constitutional Court (‘GFCC’) which was one of the primary instigators of pluralist thinking between EU law and Member State constitutional law.Footnote ⁵⁰ Perhaps most clearly, however, the ambiguity in the UKSC's reasoning in the Miller decision,Footnote ⁵¹ precisely reflects the ambiguity of co-equal constitutional authority claims and mutual accommodation captured by constitutional pluralism.

This reading of the approach of the UK courts to EU law suggests that the idea of constitutional pluralism provides a better account of the accommodation of EU law within the UK's constitutional order than the alternatives of monism or dualism.

IV. THE NEGOTIATED UNSETTLEMENT AND BREXIT

One of the fundamental constitutional changes wrought by Brexit will be the severing of the structural link between the UK constitution and the EU's legal order, and the UK courts and the EU courts, founded on the ECA. As Section 1 of the European Union (Withdrawal) Act 2018 makes clear, the ECA will be repealed on ‘exit day’. It seems then, that the pluralist relationship argued for above between EU and UK law will come to a shuddering halt when the ECA is formally repealed. We should therefore expect the future relationship to revert to an attenuated dualist one where only the parts of EU law unilaterally and explicitly implemented by the UK legislature will have effects within the UK's legal order. However, a number of features of the EUWA leave room for doubt as to the explanatory power of dualism in explaining the relationship between EU and the UK post-Brexit.

Firstly, perhaps the most striking feature of the EUWA, in repealing the ECA, is that it does not repeal all of the EU law in force in the UK prior to exit. Rather it incorporates, in dualist fashion, all EU law in force prior to exit into the UK legal system in an explicit way through the maintenance in force of existing EU law prior to exit.Footnote ⁵² Moreover, much of the EU law retained in the UK's legal system under the Act will have effects analogous to the highest form of domestic law, an Act of the UK parliament.Footnote ⁵³ The interpretation of retained EU law by the EU courts, moreover, is preserved in the EUWA such that the ‘validity, meaning or effect’ of any EU law in force in the UK legal order prior to exit is to be interpreted and applied according to the case law of the EU courts on those laws.Footnote ⁵⁴ Thus, in terms of preserving the status quo post-exit for the purposes of continuity in the immediate aftermath of the UK's exit from the EU, the EUWA creates the closest alignment possible between the EU and UK legal orders once the UK leaves the organisation.

Furthermore, a substantive factor in the development of pluralism between courts outlined above was the obligation to recognise and apply EU law in the domestic system as well as recognise the authority of the CJEU as the ultimate interpreter of EU law. The changes made to this situation in the Withdrawal Agreement (‘WA’) are significant in that it makes clear that UK courts will not be bound by CJEU jurisprudence,Footnote ⁵⁵ and cannot make preliminary rulings after exit.Footnote ⁵⁶ However, UK courts will be under an obligation to apply ‘retained’ EU law on exit day in a manner superior to pre-exit statutes for which the CJEU's case law will be binding.Footnote ⁵⁷ Even more significantly, the link between EU courts and UK courts in matters of EU law produced after exit has not been completely severed. Section 6(2) EUWA provides that:

Thus, as Section 6 makes clear, the EU courts will not be completely cut out of the UK constitution after exit. There is still a statutory link between UK domestic courts and the EU courts in the EUWA. Whereas the link is much weaker than the comparable obligation under the ECA, the fact that UK courts have a statutory obligation to have regard to ‘anything done’ by the EU courts if of relevance to ‘any matter’ before it retains a significant link between the two legal orders.

Moreover, and perhaps somewhat ironically, the EUWA in effectuating the UK's exit from the EU has gone where not even the ECA and the subsequent European Union Act 2011 dared go. For the first time, explicit statutory recognition is given to ‘the principle of the supremacy of EU law’, which will continue to apply ‘on or after exit day’ so far as relevant for applying pre-exit law.Footnote ⁵⁸ As a judicial doctrine developed by the EU courts, rather than a direct provision of treaty law,Footnote ⁵⁹ the reception of which into the UK's legal order was the product of judicial interpretation and accommodation rather than explicit statutory direction, the precise status and nature of the principle referred to in Section 5 WA has been a matter of some discussion and debate.Footnote ⁶⁰ For example, will this statutory rendering of the doctrine be ‘more Catholic than the Pope’ in that it goes above and beyond what domestic courts have been willing to ascribe to the principle in domestic law? Will it be a verbatim implementation of the principle as that intended by the EU courts? If so, how will it contend with the more recent nuanced rendering of the principle by the UK courts in recent cases such as HS2 and Miller?Footnote ⁶¹ At the very least, we can anticipate a new chapter in the accommodation of the doctrine within the UK constitution, building on its incremental development in the UK over the past number of decades. Precisely how distinctive this chapter will be, and how much it will depart from the previous understanding by the UK courts, remains to be seen.

V. CONSTITUTIONAL PLURALISM IN THE UK AFTER BREXIT

If constitutional pluralism provides a more attractive explanatory framework for the accommodation of EU law within the British constitution than its strictly monist or dualist counterparts prior to exit, then it is argued that in the light of the key features of the EUWA outlined above, it will continue to have an important explanatory power well after the UK exits the EU.

Of course, it remains to be seen precisely how these provisions of the EUWA will be interpreted but the framework it sets up is potentially even more constitutionally pluralist than the position of current EU membership. The fact that the EUWA creates a presumption in favour of the supremacy of UK law and ultimate authority of the UK courts to resolve such conflicts and claims, does not mean that they will automatically be resolved in favour of UK law in classically dualist terms, without any recognition of the validity and authority of EU law.

Firstly, we can expect the ‘living’ EU law developed by EU institutions, including the EU courts, to continue to have a role in a post-exit UK constitution. Where future EU laws or EU court judgments amend, revise, or develop areas of EU law retained in the EUWA, both during the putative ‘transition period’Footnote ⁶² as well as afterwards, we can expect it to provide a sort of ‘shadow constitution’ to the interpretation and application of EU-related law in the UK constitution for the foreseeable future.Footnote ⁶³ This is a natural result of the retention of valid EU law in the UK system on exit. Where gaps, abeyances, or ambiguities arise in the interpretation of this law, its authoritative interpretation by the EU courts cannot but have an effect on how UK courts interpret and apply retained EU, even where it seems to conflict with post-Brexit Acts of the Westminster Parliament.

Moreover, we can expect this ‘shadow constitution’ to have a significant force on the UK constitution after exit given the arrangements between the UK and EU courts set up by the WA. The types of situations which gave effect to constitutional pluralism—a system or source of law claiming authority interacting and conflicting with others claiming authority—is maintained to an extent by the structural link set up between the courts under in Section 6 of the EUWA. As noted, this provision provides that UK courts ‘may have regard’ to decisions of the EU institutions after exit provided it is ‘relevant to any matter before the court or tribunal’. The scope for taking cognisance of EU decisions is particularly broad given that it applies to ‘any matter’ before a court or tribunal and is not explicitly limited to the interpretation of application of retained EU law.Footnote ⁶⁴ Moreover, the interpretation and meaning of the term ‘have regard’ is similarly broad and therefore open to much interpretative latitude by the UK courts. In considering how broadly—and therefore pluralistically—the provision could potentially be interpreted, two factors are worth bearing in mind; the evolution in the wording of the provision in its passage through Parliament, and the way the courts have previously interpreted a similarly broad interpretative mandate in the Human Rights Act 1998 (‘HRA’). The original provision which led to Section 7 involved a more convoluted formulation using a ‘may have regard’ formula.Footnote ⁶⁵ The House of Lords Constitution Committee recommended amending the provision using stronger language; namely that courts ‘shall have regard’ to future EU court rulings.Footnote ⁶⁶ In the event, the provision was amended but the ‘may have regard’ formula retained. However, as Elliot and Tierney argue, not much turns on the distinction.Footnote ⁶⁷ The interpretation of ‘may’ will be predicated on a relevance test, such that a stronger statutory direction to have regard to future EU rulings is the intention and that therefore the structural link post-exit between UK courts and the EU courts is stronger than the use of the more discretionary term ‘may’ would suggest.

Furthermore, in considering the pluralist potential of this provision in terms of the future relationship between the courts post-exit, the experience of the HRA can provide some indication of just how broad the interpretation given—and the concomitant closer link between the courts following a broad interpretation—can be. As is well known, the HRA creates a duty on courts to ‘take into account’ ECHR case law where relevant to the case at hand.Footnote ⁶⁸ However, the interpretation of this phrase has involved quite a far-reaching emulation of the Strasbourg case law in the protection of rights in the UK.Footnote ⁶⁹ This approach reached its zenith in what became known as the ‘mirror principle’. According to the mirror principle, UK courts should follow the ‘clear and constant jurisprudence of the Strasbourg court’ unless there are ‘special circumstances’ not to do so.Footnote ⁷⁰ Thus, as its label suggests, the domestic courts should, for all intents and purposes, ‘mirror’ the rulings of the European Court of Human Rights (‘ECtHR’).Footnote ⁷¹ The significance of this from a pluralist perspective is that this interpretation sets up a particularly strong structural link between the UK courts and the ECtHR to set the stage for interactions which are inherently pluralist in character.Footnote ⁷² For example, the pluralist ‘dialogue’ that evolved out of this link was on display in the UKSC's decision in Horncastle.Footnote ⁷³ In this instance, the Court stated that there were exceptional circumstances for not following the ECtHR's case law, while suggesting that this would be:Footnote ⁷⁴

This opportunity was taken up by the ECtHR in the subsequent Al-Khawaja case, where it responded directly to the UKSC's comments in Horncastle.Footnote ⁷⁵

The point here is that notwithstanding the formal severing by the EUWA of the structural links between the EU and UK courts contained in the ECA which supported a pluralist relationship between them, the statutory licence to have ‘have regard’ to future rulings has enormous pluralist potential. Add to this the strong ‘institutional memory’ of the pre-Brexit relationship with the CJEU among the UK judiciary and we can expect this relationship to continue in pluralist terms for some time. As such, the mechanism securing the UK's departure from the EU, somewhat paradoxically secures much of the status quo during membership at least as far as the characterisation of the relationship in pluralist terms is concerned. Indeed, if anything, the post-exit UK constitutional landscape will be even more pluralist than prior to exit.

Finally, to the extent that constitutional pluralism provides an explanatory account of the internal pluralism in one legal system,Footnote ⁷⁶ it can also provide a better account of the UK's internal legal order after Brexit than dualism. Alongside the provisions setting up future EU law as a ‘shadow constitution’ post-Brexit, the EUWA essentially sets up four distinct sources of EU-related valid and enforceable law in the UK constitution: UK law passed prior to exit; EU law passed prior to exit; UK law passed after exit; and the EUWA itself. The relationship between each of these types of law—formally created according to the same constitutional authority, an Act of Parliament—will have to be negotiated and their precise constitutional status inter se unpacked in terms of some sort of hierarchy of sources, at the very least to provide certainty where their provisions conflict. In this sense, constitutional pluralism can potentially be internalised into the UK's own constitutional order with four different types of law potentially conflicting and enjoying claims to constitutional authority. Whereas the Act does make some distinctions between types of retained EU law creating some hierarchy in the relationship,Footnote ⁷⁷ this does not answer the many questions which will emerge before the courts on the different types of authority claims attached to each source. The ‘later in time’ rule applying to statutes will probably be more prominent after exit, however conflicts between the different sources of law are unlikely to be fully resolved through the application of this rule.Footnote ⁷⁸ If the justification for the creation of four distinct categories of law is certainty and continuity upon exit,Footnote ⁷⁹ it is not clear that the constitutional authority of, for example, a post-exit Parliament will necessarily and automatically trump the constitutional authority of the previous Parliament which passed the Withdrawal Act or previous pre-existing UK legislation which relates to, or attempts to incorporate EU law. This will especially be the case if a purposive interpretation is given to the EUWA and the goal of maximal legal certainty after exit becomes a prominent feature in the application of the Act. It is also likely that the ‘shadow constitution’ of the EU legal order will also have an influence in managing internal conflicts between the different sources of law identified by the EUWA, each imbued with a distinctive constitutional authority.

VI. CONSTITUTIONAL PLURALISM WITHIN THE EU POST-BREXIT

A final crucial dimension of constitutional pluralism post-Brexit is the role it will have, and continue to have, in an EU of 27 Member States. Whereas the impact of Brexit will, of course, largely be felt within the UK, the withdrawal of a large member state from the bloc does raise profound constitutional questions about the future trajectory of integration.

One implication of the post-Brexit EU political and popular landscape might be that it clears the way for further and deeper integration in a more federal direction thereby spelling the end of constitutional pluralism as a normative principle relevant to the EU. If Brexit has resulted in the greater acceptance of the project among its remaining citizens, and the inevitability of the project among its harsher eurosceptic critics, then it would seem that further developments in a more federal direction would be the natural trajectory of constitutional reform in the immediate term.Footnote ⁸⁰ Add to this the fact that one of its more awkward members is departing, and a more robust federalisation of the EU seems almost inevitable. This is certainly the interpretation of the lessons of Brexit among some of the EU's more vocal characters.Footnote ⁸¹ In this scenario, constitutional pluralism could be dispensed with, a regrettable necessity in the journey to further integration which has now outlived its purpose.Footnote ⁸²

However, it is not clear that we should dismiss the role of constitutional pluralism in future reform so swiftly. If it is the case that we can expect more favourable political winds in respect of the acceptance and popularity of the integration project for the foreseeable future among the remaining 27 Member States that is accompanied by increased popular interest and engagement with the EU, arguably constitutional pluralism is a more appropriate approach to further reform than an elite-driven federal blueprint.

Firstly, it is not clear that a bump in support for the EU such as it is can be translated into an unambiguous desire for a federal European future. Writing before the Brexit vote, Walker has argued, that there is still a lack of real social and political demand for a federal Europe,Footnote ⁸³ and it is not clear that much has changed in this regard in the Brexit aftermath. Furthermore, if support for the integration project is sustained and experiments cultivating further citizen engagement such as the Spitzenkandidaten procedure do mature into more robust democratic engagement with the EU level,Footnote ⁸⁴ including up to the level of popular constitutional reform, none of this means that a federal Europe is the inevitable and inexorable finalité politique of a post-Brexit EU.Footnote ⁸⁵ Capitalising on increased popular engagement with, and understanding of, the EU post-Brexit does not, necessarily, require a full constitutionalisation and federalisation of the project; in short an EU sovereignty.Footnote ⁸⁶ A more informed and engaged citizenry may balk at what has already been achieved at the EU level and demand some repatriation of power from the centre to the Member States. As with any multi-level governance system, especially one as contested as the EU, we can expect an emergent pan-European politics to involve good-faith disagreements about how to resolve tensions between political identity and appropriate levels of governance for the achievement of common goods alongside conventional disagreements on substantive matters of policy.Footnote ⁸⁷

In this regard, then, perhaps the ‘placeholder’ of constitutional pluralism can, as Walker suggests it should, mature into a more permanent guiding principle of EU constitutional reform. Walker ultimately calls for a ‘reconstitutionalisation’ of the EU in a resolutely pluralist direction—‘a new kind of constitutional dualism’Footnote ⁸⁸—where constitutional pluralism provides the guiding ethos of constitutional reform at the EU level such as to stabilise and enhance its autonomy, yet at the same time does not challenge its Member States as original political communities. Only once this form of genuine constitutional dualism takes hold in EU constitutional processes, he concludes, can we ‘envisage how the overlap of heterarchically related constitutional authorities of the common part and the local parts, rather than undermining or eroding the legitimacy of each such authority, becomes a condition of legitimacy of the combined whole’.Footnote ⁸⁹ The flexibility of the idea and its studied rejection of a resolution of the question of hierarchy and ultimate authority may therefore be a more apt framework principle for a fragile emerging pan-European politics, and indeed may itself become endorsed by such politics, than the federalism that many proponents of pan-European politics assume.Footnote ⁹⁰