The UK government will invoke Article 50 of the Lisbon Treaty and trigger Brexit talks on 29 March, Downing Street confirmed on Monday (20 March).
Sir Tim Barrow, the UK's chief permanent representative to the EU, has informed European Council President Donald Tusk's office of the move.
"Last June, the people of the UK made the historic decision to leave the EU," said Brexit Secretary David Davis. "Next Wednesday, the Government will deliver on that decision and formally start the process by triggering Article 50.
"We are on the threshold of the most important negotiation for this country for a generation.
"The government is clear in its aims: a deal that works for every nation and region of the UK and indeed for all of Europe – a new, positive partnership between the UK and our friends and allies in the European Union."
Once Tusk has received the letter, the Polish politician will then consult the other 27 leaders of the Council on the Brexit trigger. A response from the group is expected in around 48 hours after the UK's notification, according to European Commission negotiator Michel Barnier.
The EU leaders will later hold a summit in April or May to decide a final response. The top politicians will carve out negotiating positions and guidelines for the European Commission, the bloc's executive arm, to follow.
Jean Claude Juncker, as European Commission president, will oversee the process, while Barnier and his taskforce of negotiators deal with the UK government for the next two years. The Belgian capital of Brussels is expected to play host to most of the talks between EU and UK officials.
Barnier has said that he expects a deal to be agreed in 2018 and ratified in 2019. Pro-EU Liberal Democrat leader Tim Farron warned that May was embarking on an "extreme and divisive" Brexit plan.
"On the day 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," he said.
"She has chosen the hardest and most divisive form of Brexit, choosing to take us out of the Single Market before she has even tried to negotiate. That's why we believe the people should have the final say over the Conservative Brexit deal.
"Membership of the Single Market is vital for the British economy and for the jobs of millions of British people. Leaving the Single Market was not on the ballot paper in the referendum, it is a political choice made by Theresa May.
"Meanwhile, with the country in desperate need of an Official Opposition, Labour has declared war on itself rather than defending the people from a hard Brexit.
"You can't have a rushed Brexit and a strong, united country."