Brexit (; a portmanteau of "British" and "exit") is the withdrawal of the United Kingdom (UK) from the European Union (EU). Following a referendum held on 23 June 2016 in which 51.9 per cent of those voting supported leaving the EU, the Government invoked Article 50 of the Treaty on European Union, starting a two-year process which was due to conclude with the UK's exit on 29 March 2019 – a deadline which has since been extended to 31 October 2019.
Withdrawal from the EU has been advocated by both left-wing and right-wing Eurosceptics, while pro-Europeanists, who also span the political spectrum, have advocated continued membership and maintaining the customs union and single market. The UK joined the European Communities (EC) in 1973 under the Conservative government of Edward Heath, with continued membership endorsed by a referendum in 1975. In the 1970s and 1980s, withdrawal from the EC was advocated mainly by the political left, with the Labour Party's 1983 election manifesto advocating full withdrawal. From the 1990s, opposition to further European integration came mainly from the right, and divisions within the Conservative Party led to rebellion over the Maastricht Treaty in 1992. The growth of the UK Independence Party (UKIP) in the early 2010s and the influence of the cross-party People's Pledge campaign have been described as influential in bringing about a referendum. The Conservative Prime Minister, David Cameron, pledged during the campaign for the 2015 general election to hold a new referendum—a promise which he fulfilled in 2016 following pressure from the Eurosceptic wing of his party. Cameron, who had campaigned to remain, resigned after the result and was succeeded by Theresa May, his former Home Secretary. She called a snap general election less than a year later but lost her overall majority. Her minority government is supported in key votes by the Democratic Unionist Party.
On 29 March 2017, the Government of the United Kingdom invoked Article 50 of the Treaty on European Union. Theresa May announced the government's intention not to seek permanent membership of the European single market or the EU customs union after leaving the EU and promised to repeal the European Communities Act of 1972 and incorporate existing European Union law into UK domestic law. Negotiations with the EU officially started in June 2017. In November 2018, the Draft Withdrawal Agreement, negotiated between the UK Government and the EU, was published. The House of Commons voted against the agreement by a margin of 432 to 202 (the largest parliamentary defeat in history for a sitting UK government) on 15 January 2019, and again on 12 March with a margin of 391 to 242 against the agreement. On 14 March 2019, the House of Commons voted for the Prime Minister, Theresa May, to ask the EU for such an extension of the period allowed for the negotiation. Members from across the House of Commons rejected the agreement with the leadership of the Labour Party stating in public debates in the House of Commons that any deal must maintain a customs union and single market, and with a large percentage of its members rejecting the Irish backstop as it is currently drafted in the EU withdrawal agreement. Opponents of the EU Withdrawal Agreement cited concerns that the agreement as drafted could plunge Northern Ireland into a conflict and spark a return of The Troubles as a result of Brexit.
The broad consensus among economists is that Brexit will likely reduce the UK's real per capita income in the medium term and long term, and that the Brexit referendum itself damaged the economy. Studies on effects since the referendum show a reduction in GDP, trade and investment, as well as household losses from increased inflation. Brexit is likely to reduce immigration from European Economic Area (EEA) countries to the UK, and poses challenges for UK higher education and academic research. As of May 2019, the size of the "divorce bill"—the UK's inheritance of existing EU trade agreements—and relations with Ireland and other EU member states remains uncertain. The precise impact on the UK depends on whether the process will be a "hard" or "soft" Brexit.