EU law and Brexit

The United Kingdom's membership of the European Union and its subsequent withdrawal present the most acute modern test of the doctrine of parliamentary sovereignty. Between 1973 and 2020, EU law enjoyed supremacy within the UK legal order, requiring domestic courts to disapply inconsistent Acts of Parliament—an outcome seemingly irreconcilable with Dicey's classical understanding that no Parliament can bind its successors and that courts must give effect to the latest expression of legislative intent.

This tension was managed, rather than resolved, through a distinctive constitutional settlement. UK courts, notably in Factortame (1990), enforced EU law supremacy not as an external imposition but as the consequence of Parliament's own continuing legislative choice in the European Communities Act 1972. The withdrawal process from 2016 to 2020 vindicated orthodox sovereignty theory: Parliament could, and did, repeal the 1972 Act, unilaterally terminating the EU law supremacy regime. Yet the process also exposed the political and legal complexity of unpicking four decades of constitutional integration.

For students of constitutional law, this topic demands careful attention to doctrinal mechanics (how courts reconciled supremacy with sovereignty), statutory architecture (the ECA 1972 and the EU (Withdrawal) Act 2018), and the interplay between law and politics. It also revisits foundational concepts from earlier weeks: parliamentary sovereignty, the rule of law, the separation of powers, and the royal prerogative in relation to treaty-making. The episode is best understood not as a discrete event but as a case study illuminating the adaptive, uncodified character of the UK constitution and the limits—or absence of limits—on legislative power.

The United Kingdom joined the European Economic Community on 1 January 1973 following the passage of the European Communities Act 1972. The 1972 Act was not the product of constitutional necessity but of political choice: two previous applications (1961, 1967) had been vetoed by France. Entry required acceptance of the Community legal order, including the principles of direct effect (established by the European Court of Justice in Van Gend en Loos (1963)) and supremacy (Costa v ENEL (1964)), which held that Community law prevails over conflicting national law.

The constitutional significance was recognised at the time. The Lord Chancellor, Lord Hailsham, warned of an 'elective dictatorship' in the 1976 Dimbleby Lecture, though his concern was broader than Europe alone. Academic opinion divided: some, such as Wade, argued that ECA 1972 effected a 'technical revolution' in the UK constitution, changing the rule of recognition applied by judges. Others, including Allan and Craig, maintained that sovereignty remained intact because Parliament could always repeal the 1972 Act.

For four decades, the supremacy of EU law operated in practice. Regulations had direct effect; directives could be invoked against the state; and UK courts referred questions of EU law to the European Court of Justice under what was then Article 234 EC (now Article 267 TFEU). The legal integration deepened with each treaty amendment: the Single European Act 1986, the Maastricht Treaty 1992 (which created the European Union itself and introduced co-decision), the Amsterdam and Nice Treaties, and finally the Lisbon Treaty 2009, which gave binding force to the Charter of Fundamental Rights.

The 2016 referendum on EU membership, held pursuant to the European Union Referendum Act 2015, resulted in a 51.9% vote to leave. The advisory referendum triggered a political commitment to withdraw, but it did not itself change the law. As the Supreme Court confirmed in Miller (No 1) [2017] UKSC 5, withdrawal required an Act of Parliament because it would remove rights derived from EU law that had been conferred by statute. The formal withdrawal occurred on 31 January 2020 following the European Union (Withdrawal Agreement) Act 2020, though a transition period lasted until 31 December 2020, during which EU law continued to apply. The constitutional and legislative architecture of exit was constructed primarily by the European Union (Withdrawal) Act 2018, which repealed the ECA 1972 and sought to provide legal continuity by converting the acquis into 'retained EU law'.

Supremacy of EU law

The principle of supremacy, articulated by the ECJ in Costa v ENEL [1964] ECR 585, holds that in the event of conflict, EU law prevails over national law, including national constitutions. From the EU perspective, this is an existential necessity: without uniform supremacy, the single market and common legal order cannot function.

For the UK, supremacy posed a doctrinal puzzle. Classical sovereignty theory, as formulated by Dicey, insists that Parliament cannot bind its successors and that later statutes impliedly repeal earlier inconsistent ones. Supremacy appeared to invert this: an earlier statute (ECA 1972) would prevail over a later, inconsistent Act. Courts navigated this through construction rather than constitutional mutation. In Macarthys Ltd v Smith [1979] ICR 785, Lord Denning MR held that in case of conflict, EU law should be given priority, but suggested (obiter) that an express parliamentary statement might displace EU law. The crucial move was to treat ECA 1972 s 2(1) and s 2(4) as a continuing instruction from Parliament that EU law be given effect.

Construction, not revolution

R (Factortame Ltd) v Secretary of State for Transport (No 2) [1991] 1 AC 603

Factortame is the high-water mark of EU law supremacy in UK courts. Spanish fishermen challenged Part II of the Merchant Shipping Act 1988, which imposed nationality and residence conditions on the registration of fishing vessels, arguing that it breached EU law on freedom of establishment. The House of Lords referred the question to the ECJ, which held that national courts must set aside national law preventing the full effectiveness of EU law. The Lords then granted an interim injunction against the Crown and ultimately disapplied the 1988 Act.

Lord Bridge's judgment is constitutionally pivotal. He emphasised that the outcome was 'not in any way novel' given Parliament's acceptance of EU membership. The 1972 Act had 'precisely defined' the limits of parliamentary sovereignty for the duration of membership. Crucially, this was not judicial activism but judicial obedience to Parliament's own instruction.

Thoburn v Sunderland City Council [2002] EWHC 195 (Admin)

Laws LJ's judgment introduced the concept of 'constitutional statutes', which cannot be impliedly repealed. He identified the ECA 1972 as a constitutional statute because it conditions the relationship between the UK and EU legal orders. An ordinary statute (Weights and Measures Act 1985) could not impliedly repeal the 1972 Act; only express words or necessary implication would suffice. This doctrine reconciles supremacy with continuing sovereignty: Parliament remains sovereign because it can expressly repeal the ECA 1972, but in the absence of such repeal, EU law prevails. The judgment, though only at first instance, has been widely cited and adopted in later cases, including HS2 [2014] UKSC 3.

R (Miller) v Secretary of State for Exiting the European Union [2017] UKSC 5 (Miller No 1)

The Supreme Court held that the Crown could not trigger Article 50 TEU using prerogative powers; an Act of Parliament was required. The reasoning turned on constitutional fundamentals. Withdrawal would inevitably result in a change in domestic law (the loss of EU-derived rights), and the prerogative cannot be used to change statute law. The 1972 Act had conferred rights that would be removed by withdrawal, and only Parliament could authorise that removal.

The Court rejected the government's argument that the rights in question were contingent on the UK's continued membership and thus derived from treaty, not statute. Lord Neuberger PSC, delivering the lead judgment, held that the 1972 Act was the source of rights in domestic law. Ministers could not, by exercise of prerogative treaty-making power, remove what Parliament had enacted. The judgment reaffirmed the core separation-of-powers principle and the Diceyan rule that the Crown acts under law, not above it.

R (Miller) v The Prime Minister; Cherry v Advocate General for Scotland [2019] UKSC 41 (Miller No 2/Cherry)

This case concerned the Prime Minister's prorogation of Parliament in September 2019 for five weeks during the Brexit process. The Supreme Court unanimously held the prorogation unlawful. Lady Hale PSC, delivering the judgment, held that the prorogation had the effect of frustrating or preventing Parliament's ability to carry out its constitutional functions without reasonable justification. The court identified justiciability (the question was one of law), the legal limits on the prerogative (it cannot be used to undermine parliamentary sovereignty or responsible government), and the absence of any reasonable justification for the length of prorogation.

While Miller No 2 was not formally an 'EU law' case, it arose directly from Brexit politics and demonstrated judicial willingness to police the boundaries of executive power even in a politically charged context. It affirmed that parliamentary sovereignty includes the effective ability of Parliament to sit and legislate, and that the executive is subordinate to law.

R (on the application of Privacy International) v Investigatory Powers Tribunal [2019] UKSC 22

Though primarily a case on ouster clauses, Lord Carnwath's judgment (concurring) suggested that parliamentary sovereignty itself might be subject to judicial limitation where fundamental rule-of-law principles are at stake. This controversial dictum—that 'the courts have the last word on the interpretation of the law'—was expressly disapproved obiter by Lords Reed and Hodge in Privacy International. The debate echoes earlier dicta in Jackson v Attorney General [2005] UKHL 56, where Lords Steyn and Hope suggested that there might be constitutional limits on what Parliament can do. These statements remain controversial and have not been adopted by a majority, but they indicate ongoing judicial reflection on the scope of sovereignty in an era of constitutional stress.

European Union (Withdrawal) Act 2018 Reference [2018] UKSC (not formally reported as a case)

This was not a litigated case but an important constitutional moment: the passage of the 2018 Act through Parliament demonstrated the legislative sovereignty principle in action. Parliament could, and did, repeal the ECA 1972, terminate EU law supremacy, and redesign the domestic legal order. The political difficulty and legislative complexity should not obscure the legal simplicity: no court, no external body, and no constitutional limit prevented Parliament from legislating for withdrawal. This empirical fact vindicates the orthodox sovereignty account.

See practice questions below.

See further reading below.