- Video report by ITV News Political Correspondent Robert Peston
Theresa May will trigger EU withdrawal talks under Article 50 on March 29, Downing Street said.
The Prime Minister's letter which officially notifies the European Council of the UK's intention to quit and will set in train a two-year negotiating process expected to lead to Britain leaving the EU on March 29, 2019.
Britain's ambassador to the EU, Sir Tim Barrow, informed the office of European Council President Donald Tusk on Monday morning of the Prime Minister's plans.
Brexit Secretary David Davis said the move would initiate "the most important negotiation for this country for a generation", with the Government aiming to secure "a new, positive partnership between the UK and our friends and allies in the European Union".
Following the news, sterling dipped against the dollar and the euro.
Ms May was cleared to trigger Article 50 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.
Following her weekly session of Prime Minister's Questions on March 29, Ms May will address MPs in a statement to the Commons.
The European Commission says it is ready to begin Brexit negotiations.
In a statement they said that "everything is ready on this side", adding "we are waiting for notification".
Mr Tusk said he will present the EU 27 with draft Brexit negotiating guidelines within 48 hours of the formal triggering.
The Eu 27 are then expected to stage an extraordinary summit within four to six weeks to agree a mandate for European Commission chief negotiator Michel Barnier, with talks probably beginning in earnest in May or June.
Ms May is expected to conduct visits to all four nations of the UK before formal notifications takes place.
Speaking during a visit to Swansea on Monday, Ms May said: "I am very clear that I want to ensure we get the best possible deal for the United Kingdom that works for everyone across the United Kingdom and all parts of the UK when we enter these negotiations.
"I have set out my objectives. These include getting a good free trade deal. They include putting issues like continuing working together on issues like security at the core of what we are doing.
"We are going to be out there, negotiating hard, delivering on what the British people voted for."
Liberal Democrat leader Tim Farron slammed the Article 50 announcement, saying: "Theresa May is embarking on an extreme and divisive Brexit. She has rushed this through without a plan, and without a clue.
"On the day Theresa May is travelling the country claiming she wants to bring the United Kingdom together, she lets it be known she is about to unleash division and bitterness.
"Meanwhile, with the country in desperate need of an official Opposition, Labour has declared war on itself rather than defending the people from a hard Brexit. You can't have a rushed Brexit and a strong, united country."
Mr Farron reiterated his call for voters to have the final say on the deal negotiated by Ms May, arguing that departure from the European Single Market was not on the ballot paper in June.
The announcement of the timing of Article 50 comes after European Commission President jean-Claude Juncker warned that if Britain rejects the terms of Brexit offered by the EU - which are widely expected to include a "divorce bill" of as much as £50 billion - it may have to abandon its hopes of a trade deal.
The UK will have "the choice to eat what's on the table or not come to the table at all", Mr Juncker said.
He added he thought Brexit will bring the 27 EU member states closer together as they will "realise it's not worth leaving".
He said: "They will all see from the UK's example that leaving the EU is a bad idea. On the contrary, the remaining member states will fall in love with each other again and renew their vows with the European Union."