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. The invocation of the Lisbon Treaty article will formally begin Brexit negotiations, kicking off a two-year period during which the the and United Kingdom will try to agree the terms of Brexit and what relations will look like once Britain leaves.
"Everything is ready on this side", commission spokesman Margaritis Schinas said. "There is not going to be an early general election".
May has previously said that she would start the Brexit process "by the end of March", therefore she has met her commitment.
The UK's ambassador to the EU, Sir Tim Barrow, formally informed EU Council president Donald Tusk this morning to expect May's letter next Wednesday.
May's spokesman said the United Kingdom wanted to start withdrawal negotiations "promptly", but accepts that "it is right that the 27 have a chance to agree their position" before talks start.
But Sir Tim said such "speculation" was based on the difficulty of striking deals between countries with very different trade systems, when Britain and the European Union already had "convergence".
Last year's referendum result, she added, "was not just about leaving the EU" but was a vote for a "change in the way the country works".
David Davis, the UK Government's Brexit Secretary, hinted last week that the devolved administrations would get notice of the triggering of Article 50.
"The government is clear in its aims", he said.
The European Commission said immediately that it was ready to begin negotiations, although a source in Brussels said it would take "four to six weeks" to arrange a summit to agree a common EU position. Under this scenario, talks are likely to begin in earnest in May.
"There will need to be a longer time for the U.K.to dismember itself from the EU and a transition deal can include all exit issue plus trade", Fredrik Erixon, director of the European Centre for International Political Economy told CNBC via email.
Notification comes 279 days after the referendum of June 23 a year ago delivered a 52 per cent to 48 per cent majority in favour of withdrawal.
EU leaders have said they want to conclude the talks within 18 months to allow the terms of the UK's exit to be ratified by the UK Parliament and the European Parliament, as well as approved by the necessary majority of EU states. It is then likely to be some years thereafter before it has finalised a trade deal with the European Union, if indeed it is able to do so.
"So, that's roughly the capacity that there is in government to draft these Bills and, in terms of parliamentary time, to pass them".
The prime minister also says she wants a "phased period of implementation" of a new relationship with the European Union to give businesses time to plan. "And it is good to know that Prime Minister Theresa May shares this view".