Britain's government will begin the process of leaving the European Union on March 29, starting the clock on the two years in which to complete the most important negotiation for a generation.
Britain's ambassador to the EU, Tim Barrow, informed the European Council President Donald Tusk of the timing on Monday morning, the Department for Exiting the European Union said.
The notification of triggering Article 50 of a key EU treaty will come in the form of a letter delivered to Tusk - though it was unclear whether it would come through an actual letter or an electronic missive.
"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."
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The 10 Downing Street office of Prime Minister Theresa May said she will make a statement in the House of Commons on the day Article 50 is triggered. The announcement came after May pushed through legislation to start the negotiations to start withdrawal - a process set in motion by voters in a June 23 referendum.
Britain voted by a 52 to 48 percent majority to leave the European Union - the first member state ever to do so.
May has said she wants to leave the European single market in order to be able to control immigration.
The European Commission said it stood ready to help launch the negotiations.
"Everything is ready on this side," commission spokesman Margaritis Schinas said.
The letter May sends next week will plunge Britain into a period of intense uncertainty.
The country doesn't know what its future relationship with the bloc will look like - whether businesses will freely be able to trade, students can study abroad or pensioners will be allowed to retire with ease in other EU states. Those things have become part of life since the UK joined what was then called the European Economic Community in 1973.
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It's not even certain the United Kingdom will survive the exit intact. Scotland's nationalist first minister, Nicola Sturgeon, is seeking a referendum on independence within two years. Scottish voters backed remaining in the EU, and Sturgeon said Scotland mustn't be "taken down a path that we do not want to go down without a choice".
In Northern Ireland, which also voted to stay in the EU, the largest Irish nationalist party Sinn Fein has said it wants a referendum on splitting from the United Kingdom and uniting with the Republic of Ireland "as soon as possible".
The trigger for all this tumult is the innocuous-sounding Article 50 of the EU's Lisbon Treaty, a never-before-used mechanism for withdrawing from the bloc. It stipulates that the two sides now have until March 2019 to agree on a divorce settlement and - if possible - establish a new relationship between Britain, the world's No 5 economy, and the EU, a vast single market containing 500 million people.
With nationalism and anti-establishment, anti-immigrant sentiment spreading across Western Europe, the EU leadership in Brussels is anxious to avoid encouraging others in the 28-member bloc to bolt.
Against the backdrop of trying to keep the UK together, May has a long wish list for the EU - the closest possible trading ties, security cooperation, regaining control over immigration and restoring sovereignty in various policy areas.
The EU has baulked at her demands, saying they amount to "having your cake and eating it". May's government acknowledges its opening position is bold, and is also preparing for the possibility of crashing out of the bloc with no deal.
Source: News agencies