Britain's envoy to Brussels Tim Barrow "has this morning informed the office of European Council President Donald Tusk of the UK's intention to invoke Article 50 of the Lisbon Treaty on March 29", the Brexit ministry said in a statement.
As part of a series of visits around the UK - including to Wales, Scotland and Northern Ireland - the Prime Minister, according to Downing Street, "will be engaging and listening to people from right across the nation as we prepare to leave the European Union as one United Kingdom".
The move will formally trigger Britain's exit from the European Union following a referendum in June previous year.
U.K. Brexit Secretary David Davis has said "the first meeting, bluntly, will be about how we do this?"
The EU27 are then expected to meet in April to set early guidelines for the EU's negotiating mandate - the EU's Brexit red lines - which will then go to the European Commission, which will draw up a detailed negotiation strategy.
French diplomat Michel Barnier is the chief negotiator for the European Commission, the bloc's executive arm.
Since May will trigger the article in late March, officials said negotiations may not start until early summer "because there is now too little time to convene the 27 other European Union heads of government for a meeting in early April as Mr. Tusk had originally planned".
"The phony period is almost over, and the real work of negotiations are about to begin", McFadden said.
Scottish leader Nicola Sturgeon threw a wrench in those plans last Monday by announcing a push for a new independence referendum from the United Kingdom, which also includes England, Wales and Northern Ireland.
There's also likely to be friction over Britain's desire to maintain free trade in goods and services with the bloc, without accepting the EU's core principle of free movement of workers.
The divorce process under Article 50 gives a two-year framework for negotiations.
The British government has insisted the Brexit process is irreversible once Article 50 is triggered, although experts have said there is no legal ban on member states changing their minds before they have actually left the European Union.
May's preparations for Brexit have been hit by Scottish First Minister Nicola Sturgeon's announcement last week that she planned a new independence referendum in order to keep European Union ties.
But May has repeatedly ruled out an early vote, and on Monday her spokesman told British journalists that there was "not going to be one".
May has promised parliament a vote on the final Brexit deal, but warned that rejecting it meant Britain would leave without any agreement.