THERESA May will trigger Article 50 on 29 March beginning the two year process to take the UK out of the European Union.
Downing Street has announced the letter officially notifying the European Council of the intention to quit will set in train the negotiation process expected to lead to an exit date of 29 March 2019.
Sir Tim Barrow, Britain's ambassador to the EU, informed the office of European Council president Donald Tusk on this morning of the Prime Minister's plans.
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The announcement means May will meet her self-imposed deadline of the end of March to get the withdrawal process under way.
However, it is exactly two weeks later than the original expected date, 15 March, raising speculation the move may have been delayed after the First Minister announced plans for an independence referendum in a bid to keep Scotland in the EU and reflect the 62 per cent Remain vote north of the Border.
May 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.
The Prime Minister will address MPs in a statement to the House of Commons following her regular weekly session of Prime Minister's Questions on March 29.
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 Prime Minister to 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 per cent to 48 per cent majority in favour of withdrawal. However, only England and Wales voted in favour of leaving, with Northern Ireland joining Scotland in opposing an exit.
May, who is visiting Swansea today, is expected to visit all four UK nations 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.