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The UK's relationship with the European Union — from accession to the European Economic Community in 1973 to formal departure on 31 January 2020 — has been one of the most constitutionally significant developments of the modern era. For nearly half a century, membership placed a real and unusual limit on the central principle of the British constitution: parliamentary sovereignty. Brexit then unwound that limit, but in doing so it raised a fresh set of profound questions about the role of referendums, the status of international treaties, the relationship between the Executive, Parliament and the courts, and the durability of the devolution settlement. For Component 2 of Edexcel A-Level Politics, the EU story is indispensable: it is the single richest case study available for the demanding 30-mark essay and source questions in Section A of Paper 2, because it touches sovereignty, the judiciary, devolution, prime-ministerial power and constitutional reform all at once.
The UK joined the European Economic Community (EEC) on 1 January 1973 under the European Communities Act 1972 (ECA), passed by Edward Heath's Conservative government. The ECA was the legal mechanism that gave EU law force inside the UK: it provided that rights and obligations created by the European treaties should have direct effect in UK law, and that domestic legislation should be read and given effect subject to EU law in areas of EU competence. In other words, the ECA was the conduit through which European law flowed into the domestic legal system, and it was therefore the foundation of every constitutional consequence that followed.
It is worth emphasising the manner of accession. The UK joined by ordinary statute, not by a special constitutional procedure or a prior referendum. A confirmatory referendum was held only afterwards, in 1975, when voters were asked whether the UK should remain in the EEC; they voted to stay by roughly two to one. This sequence — join by statute, confirm by referendum — already revealed the central tension that would dominate the Brexit era: a constitution founded on parliamentary sovereignty was increasingly using popular votes to settle the largest constitutional questions.
Membership had four profound and interlocking constitutional effects.
First, the supremacy of EU law. Under doctrines developed by the European Court of Justice in cases such as Van Gend en Loos (1963) and Costa v ENEL (1964), European law took precedence over conflicting national law. In the UK this principle was given dramatic practical effect by the Factortame litigation (1990–91). Spanish fishing interests challenged the Merchant Shipping Act 1988, which had been designed to stop them registering vessels as British. The House of Lords, following a ruling of the European Court of Justice, disapplied the offending parts of the 1988 Act because they conflicted with EU law — and granted an injunction against the Crown to suspend the statute pending resolution. This appeared to breach the most basic Diceyan rule that no body can override or set aside an Act of Parliament. Factortame therefore became the textbook illustration of how membership qualified sovereignty in practice.
Second, a loss of legislative competence. In fields such as trade, the customs union, agriculture, fisheries, competition policy and much environmental regulation, the UK Parliament could not legislate inconsistently with EU law. Large areas of policy were effectively decided at the European level, through regulations that applied directly and directives that the UK was obliged to transpose.
Third, the jurisdiction of the European Court of Justice (ECJ). The ECJ had authority over the interpretation of EU law as it applied in the UK, and its rulings bound UK courts. For the first time in centuries, the highest authority on a significant body of law applying in Britain sat outside the UK.
Fourth, an additional layer of rights protection. From 2009 the EU Charter of Fundamental Rights provided rights guarantees that operated, within the scope of EU law, alongside the European Convention on Human Rights and the Human Rights Act 1998. This meant rights protection in the UK was, for a time, drawn from two distinct European sources.
Throughout membership, commentators divided sharply over what had happened to sovereignty.
| Argument: sovereignty was lost | Argument: sovereignty was pooled, not lost |
|---|---|
| Parliament could not override EU law in areas of EU competence | The UK accepted EU law voluntarily via the ECA 1972, which it could repeal at any time |
| The ECJ, not UK courts or Parliament, had the final say on EU law | Sovereignty was shared with other member states for mutual benefit, not transferred away |
| The sheer volume of EU regulation set much UK policy | The UK retained a veto in many areas and shaped EU decisions through the Council |
| Factortame showed UK statute being disapplied | The constraint was self-imposed and, as Brexit proved, reversible |
The crucial point for evaluation is that both positions contained a kernel of truth, and the distinction maps neatly onto the wider sovereignty debate. In strict legal theory, sovereignty was never surrendered: Parliament had chosen to accept the supremacy of EU law and could withdraw that acceptance whenever it wished. In day-to-day political reality, however, membership imposed a genuine and far-reaching constraint that no ordinary domestic law could match. The "pooled sovereignty" thesis was ultimately vindicated by Brexit itself, since the very fact that Parliament could repeal the ECA and leave demonstrated that the underlying sovereignty had endured throughout.
On 23 June 2016, the UK voted to leave the EU by 51.9% to 48.1% on a turnout of 72.2%. The referendum had been promised by Prime Minister David Cameron and was authorised by the European Union Referendum Act 2015. The result triggered a constitutional process — and a constitutional crisis — that dominated British politics for the next three and a half years.
The referendum raised several deep questions that recur across Section A.
Advisory versus binding. Like UK referendums generally, the 2016 vote was legally advisory: the European Union Referendum Act did not require Parliament to implement the outcome, and Parliament remained legally free to disregard it. Yet politically the result was treated as decisive. Both major parties had committed to respecting it, the government had circulated a leaflet promising to implement the decision, and the overwhelming view was that to ignore a direct popular instruction would be democratically illegitimate. This gap between the legal status of the referendum (advisory) and its political force (binding in practice) is a classic illustration of the distinction between legal and political sovereignty.
Parliamentary versus popular sovereignty. The referendum created an acute tension between the legal sovereignty of Parliament — which could in principle have ignored the result — and the democratic mandate of the people, who had been expressly asked to decide. For more than three years, MPs who personally favoured Remain wrestled with whether their duty was to their own judgement or to the instruction of the electorate. The episode introduced a powerful strand of popular sovereignty into a constitution founded on parliamentary sovereignty, and the unresolved relationship between the two lay behind much of the parliamentary deadlock of 2017–19.
The franchise. The franchise for the referendum broadly followed the Westminster parliamentary franchise: EU citizens resident in the UK (other than those of Ireland, Cyprus and Malta) and 16–17 year olds were excluded, unlike in the 2014 Scottish independence referendum, where 16–17 year olds had been able to vote. Given the magnitude and permanence of the decision, the choice of franchise was criticised as arbitrary — those most affected over the longest time horizon had least say.
In R (Miller) v Secretary of State for Exiting the EU [2017], the Supreme Court ruled, by a majority of 8 to 3, that the government could not use the royal prerogative to trigger Article 50 of the Treaty on European Union (the formal notification of an intention to leave). Only Parliament, through primary legislation, could authorise such a step. The reasoning was rooted in sovereignty: triggering Article 50 would inevitably lead to the repeal of the ECA 1972 and the removal of rights that Parliament had conferred through that statute. Because the prerogative cannot be used to alter domestic law or to strip away statutory rights, only an Act of Parliament could lawfully begin the withdrawal process.
Significance. Miller I reinforced parliamentary sovereignty by insisting that Parliament — not the Executive acting alone — must authorise the most consequential constitutional decision in a generation. It is a powerful modern example of the courts using common-law principle to police the boundary between the prerogative and statute.
Parliament responded by passing the European Union (Notification of Withdrawal) Act 2017, a short statute that authorised the Prime Minister to trigger Article 50, which she duly did in March 2017.
In R (Miller) v The Prime Minister [2019], the Supreme Court ruled unanimously (all eleven justices) that Prime Minister Boris Johnson's advice to the Queen to prorogue (suspend) Parliament for five weeks, at a critical point in the Brexit timetable, was unlawful. The Court held that the prorogation had the effect of frustrating Parliament's ability to perform its constitutional functions of legislating and holding the Executive to account, without reasonable justification. The proclamation was therefore null, and Parliament was treated as never having been prorogued.
Significance. Miller II demonstrated the judiciary's willingness to check executive power even on intensely political terrain; it established that the prerogative power of prorogation is justiciable (capable of review by the courts); and it sparked fierce debate about whether the Supreme Court had overreached into politics. The government and several commentators accused the Court of trespassing on matters that should be settled politically, and the case fed directly into later proposals to curb judicial review.
Taken together, the two Miller cases placed the Supreme Court at the very centre of the Brexit drama and, more broadly, confirmed that an uncodified constitution can still generate enforceable limits on government, supplied by judges interpreting common-law principle rather than by the words of a written charter.
A cluster of major statutes gave legal effect to withdrawal. Candidates should be able to deploy the most important of these accurately.
| Act | Year | Purpose |
|---|---|---|
| European Union (Notification of Withdrawal) Act | 2017 | Authorised the PM to trigger Article 50 |
| European Union (Withdrawal) Act | 2018 | Repealed the ECA 1972 (on exit day); converted EU law into "retained EU law" |
| European Union (Withdrawal Agreement) Act | 2020 | Implemented the Withdrawal Agreement, including the Northern Ireland Protocol |
| European Union (Future Relationship) Act | 2020 | Implemented the Trade and Cooperation Agreement |
| Retained EU Law (Revocation and Reform) Act | 2023 | Reduced the special status of retained EU law and provided for its review and reform |
The EU (Withdrawal) Act 2018 created the novel category of retained EU law — the great body of EU-derived law that was converted into domestic law on exit day, with the UK leaving formally on 31 January 2020 and the transition period ending on 31 December 2020. The purpose was legal continuity: rather than allowing decades of EU-derived rules to vanish overnight, the Act snapshotted them into UK law so that businesses and citizens were not left in a legal vacuum. The price was complexity, since retained EU law occupied an awkward status, neither fully domestic in origin nor any longer European. The Retained EU Law (Revocation and Reform) Act 2023 then began to dismantle this special status, providing for the review, revocation or reform of retained EU law and asserting Parliament's full control over what survives — a tangible demonstration of restored legislative competence.
The headline constitutional consequence of Brexit was the removal of the single most significant legal constraint on parliamentary sovereignty. Parliament is no longer bound by EU law; the ECJ no longer has general jurisdiction over UK law; and in strict legal terms Dicey's doctrine is restored in full. The "taking back control" slogan of the Leave campaign was, at root, a claim about sovereignty — the demand that final legal authority should once again rest with Westminster rather than being shared with European institutions.
It is important, however, not to overstate the restoration. Brexit removed an EU constraint, but it did not free Parliament of all external obligation. The UK remains a party to the European Convention on Human Rights (which is entirely separate from EU membership and is enforced by the Council of Europe's Strasbourg court), and it is bound by the Withdrawal Agreement and the Trade and Cooperation Agreement. These instruments create binding obligations in international law that Parliament could, in strict theory, legislate to override — but doing so would carry severe diplomatic, legal and economic consequences. So the legal sovereignty restored by Brexit continues to operate, as it always has, within a web of political and international constraint.
There is also an important nuance often missed by weaker candidates: the restoration of sovereignty was not quite total even in legal terms. Under the Withdrawal Agreement and the arrangements for Northern Ireland, the European Court of Justice retained a residual jurisdiction over the EU law that continues to apply there, and certain EU rules still bind a part of the UK directly. To that limited extent, the claim that the ECJ's jurisdiction "ended" requires qualification. A precise answer therefore distinguishes between the general end of EU supremacy across Great Britain and the partial survival of EU law and ECJ oversight in Northern Ireland — a distinction that neatly captures the difference between the rhetoric of "taking back control" and the more complicated constitutional reality that emerged.
The Northern Ireland Protocol, part of the Withdrawal Agreement, created a unique settlement for Northern Ireland to avoid a hard land border with the Republic of Ireland. In effect, Northern Ireland remained aligned with the EU's single market for goods. The constitutional implications were significant and contentious.
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