How does a country leave the European Union, and what happened when Britain did?

A member state leaves the EU by using Article 50 of the Treaty on European Union. It lets a state leave "in accordance with its own constitutional requirements": the state notifies the European Council, and the Union then negotiates a withdrawal agreement with it, taking account of its future relationship with the Union. The treaties stop applying when that agreement enters into force or, failing that, two years after the notification, unless the European Council unanimously agrees to extend the period with the state concerned.

It was a late addition. Before the Treaty of Lisbon, no treaty provision outlined a voluntary withdrawal, which made leaving technically difficult but not impossible. States about to join in 2004 pushed for an exit right, and eurosceptics in states such as the UK and Denmark pushed for the creation of Article 50. It entered into force in 2009.

The United Kingdom is the only country to have used it. Under pressure from UKIP and Eurosceptics in his party, Prime Minister David Cameron had promised a referendum. On 23 June 2016, Leave won by a slight margin, with 51.9% of the vote, while Scotland and Northern Ireland voted to remain. Theresa May triggered Article 50 on 29 March 2017.

The process took longer than the two years. Withdrawal, first scheduled for 29 March 2019, was delayed by parliamentary deadlock and three extensions. The UK left on 31 January 2020, after being a member since 1 January 1973, and a transition period then ran until 31 December 2020, during which it stayed subject to EU law and in the single market but left the EU's political bodies.