Brexit (a portmanteau of "British" and "exit") is the withdrawal of the United Kingdom (UK) from the European Union (EU). Following a UK-wide referendum in June 2016, in which 52% voted in favour of leaving the EU and 48% voted to remain a member, the UK Government, which was then led by Theresa May formally notified the EU of the country's intention to withdraw on 29 March 2017, beginning the Brexit process. The withdrawal was originally scheduled for 29 March 2019, but was then delayed by deadlock in the UK Parliament after the June 2017 general election resulted in an unexpected hung parliament, which then led to three subsequent extensions of the Article 50 process. The deadlock was only resolved after a subsequent general election was held in December 2019. Following the outcome, the UK Parliament finally ratified the withdrawal agreement, and the UK left the EU at 11 p.m. GMT on 31 January 2020. This began a transition period that is set to end on 31 December 2020, during which the UK and EU are negotiating their future relationship. The UK remains subject to EU law and remains part of the EU customs union and single market during the transition, but is no longer part of the EU's political bodies or institutions.
Withdrawal was advocated by hard Eurosceptics and opposed by pro-Europeanists and soft Eurosceptics, with both sides of the argument spanning the political spectrum. The UK joined the European Communities (EC) – principally the European Economic Community (EEC) – in 1973, and its continued membership was endorsed in the 1975 referendum. In the 1970s and 1980s, withdrawal from the EC was advocated mainly by the political left, e.g. in the Labour Party's 1983 election manifesto. The 1992 Maastricht Treaty founded the EU, but was not put to a referendum. The Eurosceptic wing of the Conservative Party led a rebellion over ratification of the treaty and, with the UK Independence Party (UKIP) and the cross-party People's Pledge campaign, pressured the Conservative prime minister David Cameron to hold a referendum on continued EU membership, which was held in June 2016. Cameron, who had campaigned to remain, resigned after the result and was succeeded by Theresa May.
On 29 March 2017, the UK government formally began the withdrawal process by invoking Article 50 of the Treaty on European Union with permission from Parliament. May called a snap general election in June 2017, which resulted in a Conservative minority government supported by the Democratic Unionist Party (DUP). UK–EU withdrawal negotiations began later that month. The UK negotiated to leave the EU customs union and single market. This resulted in the November 2018 withdrawal agreement, but the British parliament voted against ratifying it three times. The Labour Party wanted any agreement to maintain a customs union, while many Conservatives opposed the agreement's financial settlement, as well as the "Irish backstop" designed to prevent border controls between Northern Ireland and the Republic of Ireland. The Liberal Democrats, Scottish National Party (SNP), and others sought to reverse Brexit through a proposed second referendum.
On 14 March 2019, the British parliament voted for May to ask the EU to delay Brexit until June, and then later October. Having failed to get her agreement approved, May resigned as Prime Minister in July and was succeeded by Boris Johnson. He sought to replace parts of the agreement and vowed to leave the EU by the new deadline. On 17 October 2019, the British government and the EU agreed on a revised withdrawal agreement, with new arrangements for Northern Ireland. Parliament approved the agreement for further scrutiny, but rejected passing it into law before the 31 October deadline, and forced the government (through the "Benn Act") to ask for a third Brexit delay. An early general election was then held on 12 December. The Conservatives won a large majority in that election, with Johnson declaring that the UK would leave the EU in early 2020. The withdrawal agreement was ratified by the UK on 23 January and by the EU on 30 January; it came into force on 31 January 2020.
Many effects of Brexit depend on how closely the UK will be tied to the EU, or whether the transition period ends without terms being agreed (a "no-deal Brexit"). The broad consensus among economists is that Brexit will likely harm the UK's economy and reduce its real per capita income in the long term, and that the referendum itself damaged the economy. Brexit is likely to reduce immigration from European Economic Area (EEA) countries to the UK, and poses challenges for UK higher education, academic research and security. Following Brexit, EU law and the EU Court of Justice no longer have supremacy over UK laws or its Supreme Court, except to a temporary extent. The European Union (Withdrawal) Act 2018 retains relevant EU law as domestic law, which the UK could then amend or repeal.