Brexit ( or ), a portmanteau of "British" and "exit", is the impending withdrawal of the United Kingdom (UK) from the European Union (EU). It follows the referendum of 23 June 2016 when 51.9 per cent of those who voted supported withdrawal. Withdrawal has been advocated by Eurosceptics, both left-wing and right-wing, while Pro-Europeanists (or European Unionists), who also span the political spectrum, have advocated continued membership.
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 European Communities was advocated mainly by the political left, with the Labour Party's 1983 election manifesto advocating complete withdrawal. In the late 1980s, opposition to the development of the EC into an increasingly political union grew on the right, with Margaret Thatcher – despite being a key proponent of the European single market – becoming increasingly ambivalent towards Europe. 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 new UK Independence Party (UKIP) was a major advocate of a further referendum on continued membership of what had now become the European Union, and the party's growing popularity in the early 2010s resulted in UKIP being the most successful UK party in the 2014 European Parliament election. The Conservative Prime Minister David Cameron pledged during the campaign for the 2015 UK General Election to hold a new referendum—a promise which he fulfilled in 2016 following the 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. The UK is due to leave the EU on 29 March 2019 at 11 pm UK time, when the period for negotiating a Withdrawal Agreement will end unless an extension is agreed. 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. A new government department, the Department for Exiting the European Union, was created in July 2016. Negotiations with the EU officially started in June 2017, aiming to complete the withdrawal agreement by October 2018. In June 2018, the UK and the EU published a joint progress report outlining agreement on issues including customs, VAT and Euratom. In July 2018, Cabinet agreed to the Chequers plan, an outline of proposals by the UK Government. In November 2018, the Draft Withdrawal Agreement and Outline Political Declaration, agreed between the UK Government and the EU, was published. On 15 January 2019, the House of Commons voted 432 to 202 against the deal, the largest parliamentary defeat for a sitting UK government in history.
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 annual losses of £404 for the average UK household from increased inflation, and losses between 2 and 2.5 per cent of UK GDP. 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 November 2018, 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. Analysis by HM Treasury has found that no Brexit scenario is expected to improve the UK economic condition. A November 2018 Treasury publication regarding the potential impact of the Chequers proposal estimated that within 15 years the UK economy will be 3.9% worse off compared with staying in the EU.