May, who was visiting Swansea on Monday, meant to visit Northern Ireland and Scotland before the formal notification was sent by letter on 29 March, Downing Street said. The EU wants Britain to pay a hefty divorce bill - estimates have ranged up to 60 billion euros ($64 billion) - to cover pension liabilities for EU staff and other commitments the United Kingdom has agreed to.
By the end of next week, after he has received May's letter, European Council President Donald Tusk should have distributed draft guidelines for the negotiations to the 27 other national governments. A Commission spokesman said on Monday Barnier would do this "immediately" after the summit.
He said that May expected negotiations to "start promptly".
That sets a clock ticking: Article 50 says that two years from the moment of notification, "the Treaties shall cease to apply" and Britain will no longer be an European Union member. Britain hasn't ruled out a payment, but is sure to quibble over the size of the tab.
Brexit Minister David Davis said: "We are on the threshold of the most important negotiation for this country for a generation".
However Tim Farron, the leader of the Liberal Democrats whose membership has risen to record numbers on account of its appeal to "Remain" voters due to a clear pro-Europe stance, said Mrs May is "embarking on an extreme and divisive Brexit" and has "rushed this through without a plan and without a clue".
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The formal triggering will take the form of a letter from Theresa May to Tusk, Downing Street confirmed. This will then be signed off by the 27 at another European Council meeting, likely to be held around June.
Parliament's Foreign Affairs Committee has warned that the British government has not done enough to prepare for the "real prospect" that talks with the European Union may break down, ending in no deal and "mutually assured damage" to both Britain and the EU.
May has already conceded Britain will have to quit the single market for goods and services - accounting for about 44 percent of its exports - to avoid being bound by European court rulings and the free movement of migrants.
The move will begin a two-year negotiating period in which the British government and the European Union hope to agree on the terms of Britain's exit and reach a separate deal on the shape of their future relationship, most importantly on the terms of trade between the two. Britain could leave earlier if it gets a deal, and the two-year deadline can be extended if all agree.
The British government has said firmly that it will not backtrack on Brexit. But it's unclear whether Article 50 is legally reversible. She will notify President Tusk in writing. "You can change your mind while the process is going on". May will probably take her cue from a catchphrase of predecessor Margaret Thatcher: "The lady's not for turning".