Brexit: The United Kingdom's Withdrawal from the European Union (2016–2020)

Brexit — the withdrawal of the United Kingdom from the European Union — was the first exercise of Article 50 TEU, the withdrawal clause introduced by the Treaty of Lisbon. The process, spanning from the UK referendum of 23 June 2016 to the entry into force of the Trade and Cooperation Agreement on 1 January 2021, fundamentally restructured the relationship between the UK and the EU, tested the resilience of the EU legal order, and generated an unprecedented volume of litigation before UK courts, the Court of Justice, and international tribunals. Brexit was the first time a Member State voluntarily left the EU, and its legal and political consequences continue to shape the development of EU external relations law.

The Referendum and Article 50 Notification

The UK referendum on EU membership was held on 23 June 2016 under the European Union Referendum Act 2015. The referendum produced a 51.89 per cent vote in favour of leaving the EU and a 48.11 per cent vote in favour of remaining, on a turnout of 72.2 per cent. The referendum was advisory under UK constitutional law, but the government committed to implementing the outcome. David Cameron resigned as Prime Minister immediately after the result, and Theresa May succeeded him on 13 July 2016.

The Article 50 notification — the formal communication to the European Council of the UK’s intention to withdraw — was the subject of the landmark UK constitutional case R (Miller) v Secretary of State for Exiting the European Union [2017] UKSC 5. The UK Supreme Court held that the government could not trigger Article 50 without an Act of Parliament authorising the notification, because withdrawal would alter domestic law by removing EU-derived rights and could not be achieved through the royal prerogative. The European Union (Notification of Withdrawal) Act 2017 received royal assent on 16 March 2017, and the UK notified the European Council of its intention to withdraw on 29 March 2017.

The Withdrawal Agreement

The Withdrawal Agreement — the treaty governing the UK’s orderly withdrawal from the EU — was the product of over three years of negotiation. The Agreement covers: citizens’ rights (protecting EU citizens in the UK and UK citizens in the EU who exercised free movement rights before the end of the transition period); the financial settlement (the UK’s contribution to EU budget commitments, estimated at £35-39 billion); the transition period (until 31 December 2020, during which EU law continued to apply to the UK); governance and dispute resolution (a Joint Committee and binding arbitration); and the Protocol on Ireland and Northern Ireland.

The Withdrawal Agreement was approved by the European Parliament on 29 January 2020 and concluded by the Council on 30 January 2020. The UK ratified the Agreement through the European Union (Withdrawal Agreement) Act 2020. The Agreement entered into force on 31 January 2020, and the UK ceased to be a Member State at 11 pm GMT that evening. The transition period operated until 31 December 2020.

The Protocol on Ireland and Northern Ireland

The Protocol on Ireland and Northern Ireland was the most contentious element of the Withdrawal Agreement. The Protocol’s objective was to avoid a hard border on the island of Ireland while ensuring the integrity of the EU’s single market. The Protocol achieved this by keeping Northern Ireland aligned with EU single market rules for goods, applying EU customs legislation to Northern Ireland, and requiring customs controls on goods moving from Great Britain to Northern Ireland (the Irish Sea border). Northern Ireland remained in the UK’s customs territory but applied EU customs rules, creating a hybrid legal status.

The Protocol generated intense political controversy in the UK, particularly among Unionist parties in Northern Ireland who argued that it undermined Northern Ireland’s constitutional position within the UK. The UK government introduced the Northern Ireland Protocol Bill in June 2022, which would have empowered UK ministers to disapply parts of the Protocol unilaterally. The Bill was controversial in international law and was not enacted. The Protocol was replaced by the Windsor Framework agreed between the UK and the EU on 27 February 2023, which introduced new arrangements for customs procedures, agri-food trade, VAT, and medicines, and established the “Stormont Brake” mechanism allowing the Northern Ireland Assembly to object to new EU goods rules.

The Trade and Cooperation Agreement

The Trade and Cooperation Agreement (TCA) , concluded on 24 December 2020 and provisionally applied from 1 January 2021, governs the future relationship between the UK and the EU. The TCA is a comprehensive free trade agreement providing for zero tariffs and zero quotas on all goods that comply with rules of origin requirements. The Agreement includes provisions on: trade in goods and services; digital trade; intellectual property; public procurement; aviation and road transport; energy; fisheries; social security coordination; and law enforcement and judicial cooperation in criminal matters.

The TCA does not replicate the depth of integration that existed when the UK was a Member State. It contains no mutual recognition of services regulation, no automatic recognition of professional qualifications, and no participation of the UK in EU programmes (with limited exceptions). The Agreement includes a level playing field chapter with commitments on state aid, competition, social and employment standards, environment, and climate change, enforceable through rebalancing measures and binding arbitration. The TCA is governed by a Partnership Council and specialised committees.

Article 50 Litigation

The Article 50 process generated significant litigation. In Wightman v Secretary of State for Exiting the European Union (Case C-621/18), the Court of Justice held that the UK could unilaterally revoke its Article 50 notification, restoring its status as a Member State, provided the revocation was decided in accordance with UK constitutional requirements. The judgment confirmed that Article 50 is a voluntary process and that revocation is possible, though the UK chose not to exercise this option.

The Withdrawal Agreement and TCA generated disputes before the Joint Committee (including the UK’s invocation of Article 16 of the Northern Ireland Protocol in November 2022, which was rapidly withdrawn) and arbitral tribunals. The UK’s post-Brexit legislation, including the Retained EU Law (Revocation and Reform) Act 2023, which sunsetted most retained EU law at the end of 2023 (later amended), continues to shape the evolution of UK law after departure.