The EU, Brexit, and Constitutional Implications

The UK's relationship with the European Union — from accession in 1973 to departure in 2020 — has been one of the most constitutionally significant issues of the modern era. Brexit raised fundamental questions about parliamentary sovereignty, the role of referendums, the status of international treaties, and the relationship between the Executive, Parliament, and the courts.

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The UK and the EU: A Constitutional History

Joining the EEC (1973)

The UK joined the European Economic Community (EEC) on 1 January 1973 under the European Communities Act 1972 (ECA). This Act gave EU law direct effect in UK law and established the supremacy of EU law over domestic legislation in areas of EU competence.

The Constitutional Impact of EU Membership

EU membership had profound constitutional effects:

1. Supremacy of EU law — Under the doctrine established in Costa v ENEL (1964) and Van Gend en Loos (1963), EU law took precedence over national law. In the UK, this was given practical effect by the Factortame case (1990), in which the House of Lords disapplied parts of the Merchant Shipping Act 1988 because they conflicted with EU law. This appeared to breach the Diceyan principle that no body can override an Act of Parliament.

2. Loss of legislative competence — In areas such as trade, agriculture, fisheries, and environmental regulation, the UK Parliament could not legislate inconsistently with EU law.

3. The European Court of Justice (ECJ) had jurisdiction over the interpretation of EU law in the UK, and its rulings were binding on UK courts.

4. The Charter of Fundamental Rights (from 2009) provided an additional layer of rights protection alongside the ECHR and the HRA.

The Sovereignty Debate During Membership

Those who argued sovereignty was lost:

• Parliament could not override EU law in areas of EU competence.
• The ECJ had the final say on matters of EU law.
• The volume of EU regulation meant that significant areas of policy were determined at the European level.

Those who argued sovereignty was retained (pooled):

• The UK voluntarily accepted EU law through the ECA 1972; it could repeal the Act at any time.
• Sovereignty was pooled, not transferred — shared with other member states for mutual benefit.
• The UK retained a veto in many policy areas and could influence EU decisions through the Council of Ministers and the European Council.
• Brexit ultimately proved that sovereignty had not been permanently lost.

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The EU Referendum (2016)

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 was called by Prime Minister David Cameron under the European Union Referendum Act 2015.

Constitutional Issues Raised by the Referendum

1. Advisory vs binding: UK referendums are legally advisory — Parliament is not legally obliged to implement the result. However, both major parties committed to respecting the result, and the political pressure to do so was overwhelming.

2. Parliamentary sovereignty vs popular sovereignty: The referendum created a tension between the legal sovereignty of Parliament (which could ignore the result) and the democratic mandate of the people (who had been asked to decide).

3. The franchise: EU citizens resident in the UK and 16–17 year olds were excluded from the franchise (unlike in the 2014 Scottish independence referendum). This was criticised as an arbitrary limitation on participation in a decision of historic magnitude.

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The Article 50 Process and the Miller Cases

Miller I (2017)

In R (Miller) v Secretary of State for Exiting the EU [2017], the Supreme Court ruled (by 8–3) that the government could not use the royal prerogative to trigger Article 50 (notification of intention to leave the EU). Only Parliament could authorise such a fundamental constitutional step, because triggering Article 50 would inevitably lead to the repeal of the ECA 1972 and the removal of rights created by EU law.

Significance: The case reinforced parliamentary sovereignty by ensuring that Parliament — not the Executive — authorised the most consequential constitutional decision in a generation.

Parliament subsequently passed the European Union (Notification of Withdrawal) Act 2017, authorising the PM to trigger Article 50.

Miller II / The Prorogation Case (2019)

In R (Miller) v The Prime Minister [2019], the Supreme Court unanimously ruled that Boris Johnson's advice to the Queen to prorogue Parliament for five weeks was unlawful. The prorogation had the effect of preventing Parliament from carrying out its constitutional functions at a critical time.