The European Union has spelled out what Britain must do to remove itself from the bloc following Thursday's referendum.
It said that Article 50, which sets a two-year deadline for a deal, can be triggered by the government through a formal declaration either in a letter or a speech.
"It has to be done in an unequivocal manner with the explicit intent to trigger Article 50," a spokesman for the European Council told the BBC.
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"It could either be a letter to the president of the European Council or an official statement at a meeting of the European Council duly noted in the official records of the meeting."
It comes as the Labour party has been plunged into crisis following Britain's decision to leave the EU.
Shadow foreign secretary, Hilary Benn, was sacked by Jeremy Corbyn late last night and shadow health secretary, Heidi Alexander, stepped down this morning.
Leave clinched a victory by 52 per cent to 48 per cent in the referendum. The turnout stood at 71 per cent, the highest UK-wide vote since the 1992 general election.
Over two million people have signed a petition for a second referendum. However, experts have branded the move a "waste of time".