Brexit (; a portmanteau of "British" and "exit") is the withdrawal of the United Kingdom (UK) from the European Union (EU), which is scheduled to take place on 29 March 2019 at 11 pm UK time, when the period for negotiating a Withdrawal Agreement will end unless an extension is agreed. There is an ongoing debate about leaving with a Withdrawal Agreement that has been ratified by both parties as an international treaty between UK and EU or leaving with no such treaty. On 14 March 2019, the House of Commons voted by a majority of 211 votes for the Prime Minister, Theresa May, to ask the EU for such an extension of the period allowed for the negotiation. Brexit follows the referendum of 23 June 2016 when 51.9 per cent of voters chose to leave the EU. Withdrawal has been advocated by Eurosceptics, both left-wing and right-wing, while pro-Europeanists, 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. 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 cross-party People's Pledge campaign was also influential in bringing about a referendum. 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. 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, 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 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 in history for a sitting UK government. A second vote on 12 March was voted against 391 to 242, a loss of 149 votes, 81 less than the vote in January.
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 had 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 February 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. Analysis by HM Treasury has found that there is no Brexit scenario that is expected to improve the UK economic condition.