Here is a timeline of the key moments so far in the UK's exit from the European Union:
January 23 - Prime Minister David Cameron promises an in-out referendum on EU membership if the Conservatives win the 2015 general election.
May 7 - The Tories unexpectedly secure a Commons majority and the PM vows to deliver his manifesto pledge for a referendum.
June 23 - The UK votes to leave the EU in a shock result that saw 52% of the public support Brexit and Mr Cameron resigns saying it would not be right to "try to be the captain that steers our country to its next destination".
July 13 - Theresa May, who backed Remain, takes over as Prime Minister and promises to "rise to the challenge" of negotiating the UK's exit.
November 10 - The High Court rules against the Government and says Parliament must hold a vote to trigger Article 50, the mechanism that begins the exit from the EU.
January 17 - Mrs May gives a speech at Lancaster House setting out her 12 key objectives for EU withdrawal and warning that "no deal for Britain is better than a bad deal for Britain".
March 29 - The Prime Minister triggers Article 50 in a letter to European Council president Donald Tusk.
June 8 - Mrs May loses her government's majority after calling a snap general election.
June 19 - Brexit Secretary David Davis and European Commission chief negotiator Michel Barnier open negotiations in Brussels.
September 22 - Mrs May makes a speech in Florence saying the UK could continue to be subject to existing EU rules until 2021 and pledging other EU nations will not be left out of pocket by Britain's decision to pull out.
March 23 - Leaders of the 27 remaining EU states clear the way for talks on Britain's future trade and security relations with the bloc.