The UK's ambassador to the EU, Sir Tim Farrow has informed the EU Council of the news, which will officially start the Brexit process.
The move comes nine months after Britain voted 51.9 per cent to 48.1 per cent in favour of Brexit in a referendum on June 23, 2016. European Union leaders plan an initial response within two days of May triggering Article 50, before convening a summit in late April or early May to ratify guidelines for their chief Brexit negotiator, Michel Barnier.
The Brexit secretary said Britain will then be engaged in the most important negotiation "for a generation".
If she meets here goal of completing negotiations within the two-year Article 50 period, then it will mean Britain leaves the European Union on March 29 2019.
Asked if that would mean Brexit happening by March 29 2019, the PM's spokesman said: "We have said we expect this to be a two-year process and we are confident that is what we will achieve". She circled "by the end of March".
Prime Minister Theresa May has vowed for months that the country will trigger Article 50 by the end of March.
However, the negotiating game will now begin, and attention will be paid to whether there is any reaction on foreign exchange markets in particular, with sterling tending to weaken on any indications of a hard Brexit.
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.
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The European Commission, whose chief negotiator Michel Barnier will spearhead the talks with London on behalf of the other 27 member states, said it was ready for the Brexit process.
Speaking soon after the date was announced during her ongoing tour of Wales, the British Prime Minister said she would work towards getting a "good free trade deal" from the EU.
He added: "Fundamentally though, you rightly say, we don't know what is going to come out of these negotiations, at all".
Jeroen Dijsselbloem, president of the finance ministers of the 19-country euro currency area, said he wanted "realism" from May's government.
Her latest visit was in the driving rain of Swansea Bay as she bookends her pre-article 50 period in Downing Street with another series of visits to the devolved nations of the UK. This was approved by Parliament two weeks ago when the Brexit bill was passed, giving the prime minister the power to get the procedure started. 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".
"Which is why if I was in the position that Theresa May was in, I would not be giving up on membership of the single market".