An EU spokesman said it was "ready and waiting" for the letter.
Mrs May's spokesman appeared to rule out an early general election at the same time as revealing the date of Brexit, saying: "There is not going to be one". We don't know if Britain could change its mind, withdraw its Article 50 notification and stay after all.
The formal process is expected to take two years according to an official table setting out deadlines, which - if stuck to - would see the United Kingdom officially become autonomous in 2019.
President of the European Council Donald Tusk has said he will issue draft Brexit negotiating guidelines to the other member states within 48 hours of the United Kingdom triggering article 50; in other words, before the end of next week.
In order to do this the British government would have to invoke Article 50 of the EU's Lisbon Treaty.More news: Comey says 'no information' to support Trump wiretapping claims
The British government confirmed that it would trigger Article 50 on March 29th, 2017.
We don't know if other European Union countries would agree to Britain going back to renegotiate the deal.
Sterling fell half a cent against the dollar on what Brexit minister David Davis described as a move taking Britain to "the threshold of the most important negotiation for this country for a generation".
Speaking ahead of the visit, Ms May said: "From my first day on the steps of Downing Street, I made clear my determination to strengthen and sustain the precious union".
Mr Tusk said recently that European Union leaders would respond within "more or less 48 hours" of the letter, starting possibly the most complicated series of negotiations in British history. The Government will want due recognition of Ireland's priorities in the initial negotiating position. Under this scenario, talks are likely to begin in earnest in May.More news: Nicki Minaj & Ty Dolla $ign
Sources said European Union leaders would not be ready to tackle the issue at a previously planned summit on 6 April, as it would take four to six weeks to get ready after they received the UK's notification. "There is a timetable that everyone has bought into it".
The Prime Minister will give a statement to MPs in the House of Commons next week, after formally pulling the Brexit trigger. European leaders have drawn a tough line, signaling they will not allow Britain to enjoy the benefits of EU membership but not bear the responsibilities.
She did not say if she will veto Sturgeon's request.
Barnier has said that he expects a deal to be agreed in 2018 and ratified in 2019.
"It is also extraordinary that the prime minister has failed to provide any certainty about her plans for Brexit or to prepare for the clear dangers of not reaching a deal with the EU".More news: Congress awaits FBI, NSA testimony on Russia-Trump links
July. 11: Following a heated leadership contest, Home Secretary Theresa May becomes Prime Minister-elect when her competitors withdraw from the race. Mrs May is not attending the event.
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