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.
Mrs May is expected to make a statement to the House of Commons on Wednesday shortly after invoking Article 50, setting out her aims.
Asked if the devolved governments had been informed this notification was being given on Monday, the Prime Minister's official spokesman said: "We are informing you all essentially now".
May has said she wants to leave the European single market in order to be able to control immigration.
After May's letter reaches Tusk, he is expected to distribute draft guidelines for the negotiations to the 27 other European Union member states.
Downing Street failed to tell Scottish ministers it would trigger Article 50 before making a public announcement, despite previous suggestions.
"They will all see from the UK's example that leaving the European Union is a bad idea", Juncker said.
"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.
Donald Tusk, president of the European Council, reacted to the announcement on Twitter by saying, "Within 48 hours of the United Kingdom triggering Article 50, I will present the draft Brexit guidelines to the EU27 Member States".
That summit will draw up a mandate for the European Commission's chief negotiator, Michel Barnier, probably allowing talks to begin in earnest in May.
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.
It came as a new think tanks report reveals bills to manage Brexit will consume Parliament entirely from May onwards leaving "very little time for anything else".
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".
Labour leader Jeremy Corbyn said: "The Government has failed to build a consensus about what form Brexit should take".
"It will be a challenge for both the government and Parliament to do all this while still ensuring full scrutiny and leaving room for the government's domestic policy agenda".
It states that any exit deal must be approved by a "qualified majority" (72% of the remaining 27 European Union states) and must also get the backing of MEPs.