Britain's government will begin the process of leaving the European Union on March 29, starting the clock on two years in which to complete the most important negotiation for a generation.
Britain's permanent representative to the EU, Tim Barrow, informed the European Council President Donald Tusk of the timing on Monday morning, the U.K.'s Department for Exiting the European Union said in a statement.
The notification of triggering Article 50 will come in the form of a letter.
"We are on the threshold of the most important negotiation for this country for a generation," Brexit secretary David Davis said. "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."
Brexit Secretary David Davis said in a statement that the country is "on the threshold of the most important negotiation" for a generation.
Britain voted 52 per cent to 48 per cent to leave the European Union in a referendum on June 23.
Here's a look at the main issues and what happens next:
WHAT IS THE EU AND WHY IS BRITAIN LEAVING?
The EU is a bloc of 28 nations sharing relatively open borders, a single market in goods and services and, for 19 nations, a single currency, the euro.
Britain joined in 1973, but has long been a somewhat reluctant member, with a large contingent of euroskeptic politicians and journalists regularly railing against regulations imposed by EU headquarters in Brussels.
Former Prime Minister David Cameron offered voters a referendum on EU membership, and in June they voted by 52-48 per cent to leave.
HOW DOES BRITAIN FILE FOR DIVORCE?
The British government will invoke Article 50 of the EU's Lisbon Treaty, which says a member state may "notify the European Council of its intention" to leave the bloc.
The Department for Exiting the European Union says notification will come in a letter from May to European Council President Donald Tusk. May will also announce the news in Parliament.
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 EU member.
WHOSE MOVE IS IT NOW?
The timing of Article 50 was up to Britain. What happens next is up to the EU.
European Commission spokesman Margaritis Schinas said Monday that the EU is "ready to begin negotiations."
Tusk has said that that once EU officials get Britain's notification, they will respond within 48 hours, offering draft negotiating guidelines for the 27 remaining member states to consider. Leaders of the 27 nations will then meet in April or May to finalize their negotiating platform.
U.K. Brexit Secretary David Davis has said "the first meeting, bluntly, will be about how we do this? How many meetings, you know, who's going to meet, who's going to come."
Substantial talks may have to wait until after France's two-round April-May election for a new president. Another hiccup could be Germany's September election, which will determine whether Chancellor Angela Merkel gets another term.
WHO CONDUCTS THE NEGOTIATIONS?
On the British side, Davis will take the lead, reporting to May. Britain's ambassador to the EU, Tim Barrow, will also play a major role, and the Foreign Office will talk to individual member states to try to get them on its side.
On the EU side, it's complicated. As Britain's Institute for Government recently pointed out, "the U.K. is negotiating with 27 member states, not a unified bloc."
French diplomat Michel Barnier is the chief negotiator for the European Commission, the bloc's executive arm. He'll receive direction from the Council, which represents the leaders of the member states.
The European Parliament also wants a say, and will have to approve the final deal between Britain and the bloc.
WHAT IS THE MOST PRESSING ISSUE?
Britain's vote to leave the EU has meant uncertainty for 3 million EU citizens living in the U.K., and 1 million Britons who reside in the 27 other nations of the bloc. Both sides agree that giving such citizens a guarantee that they will be able to stay where they are is a top priority.
WHAT WILL BE THE MAIN CONFLICTS?
The first major battle is likely to be about money. The EU says Britain must pay a hefty divorce bill of up to 60 billion euros ($64 billion), to cover EU staff pensions and other expenses the U.K. has committed to. Britain hasn't ruled out a payment, but is sure to quibble over the size of the tab.
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. Britain has said it will impose limits on immigration, and so will have to leave the EU's single market and customs union. That makes some barriers to trade seem inevitable.
WHEN WILL IT BE OVER?
Under the terms of Article 50, Britain will cease to be an EU member in March 2019.
But EU negotiators warn it could take two years just to settle the divorce terms; agreeing on a new relationship for the U.K. and the EU could take years longer. If the rest of the EU agrees, the two-year negotiating period can be extended, leaving Britain in the EU for a while longer. Or, the two sides could agree on a transitional period.
There's also a chance Britain could walk away early without a deal if it thinks the talks are going nowhere.
IS BREXIT A ONE-WAY TICKET?
The British government has said firmly that it will not backtrack on Brexit. But it's unclear whether Article 50 is legally reversible. Former British ambassador to the EU John Kerr, who wrote Article 50, says "it is not irrevocable. You can change your mind while the process is going on."
However, domestic political pressures make it unlikely that the British government would try a U-turn. May will probably take her cue from a catchphrase of predecessor Margaret Thatcher: "The lady's not for turning."