British Prime Minister Theresa May will trigger EU withdrawal talks under Article 50 on March 29, Downing Street has announced.
Her letter officially notifying the European Council of the UK's intention to quit will set in train a two-year negotiation process expected to lead to Britain leaving the EU on March 29, 2019.
Britain's ambassador to the EU, Tim Barrow, informed the office of European Council president Donald Tusk morning of the Prime Minister's plans this morning.
The announcement means that Mrs May will meet her self-imposed deadline of the end of March to get the withdrawal process underway.
She was cleared to take the step when the European Union (Notification of Withdrawal) Act gained royal assent last week, after a Supreme Court ruling forced her to seek the approval of both Houses of Parliament.
Mrs May will address MPs in a statement to the House of Commons following her regular weekly session of Prime Minister's Questions on March 29.
Mr Tusk has previously said he expects to release an initial response to the Article 50 notification within 48 hours, and an extraordinary summit of the remaining 27 EU member states is due to be called within four to six weeks.
That summit will draw up a mandate for the European Commission's chief negotiator, Michel Barnier, probably allowing talks to begin in earnest in May.
Notification of the historic step will come in the form of a letter from the British Prime Minister to Mr Tusk, though Downing Street did not make clear whether this would be a physical letter handed to the European Council president by a UK representative or might be sent electronically.
It will be the first time that the provisions of Article 50 - which sets out the process for any EU member state "to withdraw from the Union in accordance with its own constitutional requirements" - have been activated.
Notification comes 279 days after the referendum of June 23 last year delivered a 52%-48% majority in favour of withdrawal.
Mrs May, who was visiting Swansea on Monday, is expected to conduct visits in all four nations of the UK before notification takes place.
The PM's official spokesman said: "Earlier this morning, the UK Permanent Representative to the EU informed the office of Donald Tusk that it is the UK's intention to trigger Article 50 on March 29.
"There will be a letter. She will notify President Tusk in writing. The Prime Minister will give a statement to Parliament as well.
"We have always been clear that we will trigger by the end of March and we have met that timetable."
The spokesman said Britain wanted to start withdrawal negotiations "promptly", but accepts that "it is right that the 27 have a chance to agree their position" before talks start.