Image copyright PA Image caption Theresa May signs the letter to the EU confirming the UK's departure 
 
 Theresa May has signed the letter that will formally begin the UK's departure from the European Union. 
 
 Giving official notice under Article 50 of the Lisbon Treaty, it will be delivered to European Council president Donald Tusk later. 
 
 In a statement in the Commons, the prime minister will then tell MPs this marks "the moment for the country to come together". 
 
 It follows June's referendum which resulted in a vote to leave the EU. 
 
 Mrs May's letter will be delivered at 12:20 BST on Wednesday by the British ambassador to the EU, Sir Tim Barrow. 
 
 Image copyright AFP Image caption Sir Tim arrived in Brussels earlier clutching a briefcase containing the Article 50 letter 
 
 The prime minister, who chaired an hour long cabinet meeting earlier, has left Downing Street to make a statement to MPs confirming the countdown to the UK's departure from the EU is under way. 
 
 She will promise to "represent every person in the whole United Kingdom" during the negotiations - including EU nationals, whose status after Brexit has yet to be settled. 
 
 "It is my fierce determination to get the right deal for every single person in this country," she will say. 
 
 "For, as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can - and must - bring us together." 
 
 Attempting to move on from the divisions of June's referendum, Mrs May will add: "We are one great union of people and nations with a proud history and a bright future. 
 
 "And, now that the decision has been made to leave the EU, it is time to come together." 
 
 Labour leader Jeremy Corbyn said his party respected the decision to leave the EU and would hold the government to account "every step of the way". 
 
 He said: "Britain is going to change as a result. The question is how." 
 
 Mr Corbyn warned it would be "a national failure of historic proportions" if Mrs May does not secure protection for workers' rights. 
 
 Chancellor Philip Hammond said the triggering of Article 50 was "a pivotal moment for Britain" and insisted the government "will get a deal". 
 
 But he suggested on BBC Radio 4's Today programme that ministers would be prepared to compromise during the process, adding: "Everybody in the EU and the UK is going to go into this negotiation looking to protect their own interests... 
 
 "We understand that we can't cherry-pick, we can't have our cake and eat it - that by deciding to leave the EU and negotiate a future relationship with the EU as an independent nation, there will be certain consequences of that and we accept those." 
 
 He said EU citizens could still move to the UK and have their full rights while the UK remains "full members of the EU for the next two years". 
 
 But crossbench peer Lord Gus O'Donnell, formerly Britain's top civil servant, likened the triggering of Article 50 to being "in a plane being flown by members of the EU and we're about to jump out and we have got a parachute designed by the people flying the plane - and they have designed it in a way to deter anyone else jumping out". 
 
 Media playback is unsupported on your device Media caption Brexit divorce: 'Conscious uncoupling' or bitter breakup? 
 
 Media playback is unsupported on your device Media caption Seven decades of the UK in Europe 
 
 The PM will be outlining the next steps during a special half hour BBC One interview on Wednesday with Andrew Neil at 19:00 BST on "Britain after Brexit", to mark the triggering of Article 50. 
 
 On Tuesday night, Mrs May spoke by telephone to Mr Tusk, EU Commission president Jean-Claude Juncker and German Chancellor Angela Merkel. 
 
 Article 50 gives both sides two years to reach agreement, so unless both sides agree to extend the deadline for talks, the UK will leave on 29 March 2019. 
 
 Negotiations are expected to begin in mid-May. The UK government says it wants to carry out both separation and trade talks at the same time, but EU chiefs say the two issues must be handled separately. 
 
 The UK has said it wants an "early agreement" to guarantee the rights of EU citizens living in the UK and those of British nationals living abroad. 
 
 Other issues which are likely to be discussed are things like cross-border security arrangements, the European Arrest Warrant, moving EU agencies which have their headquarters in the UK and the UK's contribution to pensions of EU civil servants - part of a wider "divorce bill" which some reports have suggested could run to £50bn. 
 
 Up for discussion 
 
 Trade: The UK will withdraw from the single market and seek a new customs arrangement and a free trade agreement with the EU 
 
 The UK will withdraw from the single market and seek a new customs arrangement and a free trade agreement with the EU Expats: The government wants to secure an agreement with European countries "at the earliest opportunity" on the rights of EU nationals in the UK and Britons living in Europe 
 
 The government wants to secure an agreement with European countries "at the earliest opportunity" on the rights of EU nationals in the UK and Britons living in Europe Brexit bill: The UK government has promised to honour its obligations as it leaves, but has brushed off claims these could run to £50bn 
 
 The UK government has promised to honour its obligations as it leaves, but has brushed off claims these could run to £50bn Northern Ireland border: Aiming for "as seamless and frictionless a border as possible between Northern Ireland and Ireland" 
 
 Aiming for "as seamless and frictionless a border as possible between Northern Ireland and Ireland" Sovereignty: Britain will leave the jurisdiction of the European Court of Justice but seek to set up separate resolution mechanisms for things like trade disputes 
 
 Britain will leave the jurisdiction of the European Court of Justice but seek to set up separate resolution mechanisms for things like trade disputes Security: The UK government has said it wants to continue to cooperate on security and intelligence-sharing 
 
 The UK government has said it wants to continue to cooperate on security and intelligence-sharing Transitional deal: An interim arrangement may be needed before the final arrangements come into force 
 
 The Lib Dems claimed Mrs May was "pulling the trigger that will set in motion a chain of events which will change this country forever, and doing so without a proper plan", but the Leave Means Leave campaign congratulated her on sticking to her timetable of invoking Article 50 before the end of March. 
 
 "Unpatriotic, pro-EU fanatics will continue to try to derail or, at the very least, delay Brexit," the group's co-chairman, Richard Tice, warned. 
 
 The PM was forced to consult Parliament before invoking Article 50 after it lost a legal challenge in the Supreme Court, but it secured the backing of most MPs earlier this month. 
 
 On Thursday the government is expected to publish details of its "Great Repeal Bill", which aims to convert EU law into domestic legislation and repeal the European Communities Act, which says EU law is supreme to the UK's. 
 
 Get news from the BBC in your inbox, each weekday morning ||||| Britain's Prime Minister Theresa May, sitting below a painting of Britain's first Prime Minister Robert Walpole, signs the official letter to European Council President Donald Tusk, in 10 Downing Street,... (Associated Press) 
 
 Britain's Prime Minister Theresa May, sitting below a painting of Britain's first Prime Minister Robert Walpole, signs the official letter to European Council President Donald Tusk, in 10 Downing Street, London, Tuesday March 28, 2017, invoking Article 50 of the bloc's key treaty, the formal start of... (Associated Press) 
 
 LONDON (AP) — Britain is set to formally file for divorce from the European Union Wednesday, walking out on a 44-year relationship, enacting the decision made by U.K. voters in a referendum nine months ago and launching both Britain and the bloc into uncharted territory. 
 
 Prime Minister Theresa May is due to tell House of Commons at lunchtime that she has invoked Article 50 of the EU's key treaty, the trigger for a two-year countdown to Britain's exit. 
 
 Just before May's statement, scheduled for 1130 GMT (7:30 a.m. EDT), Britain's EU envoy, Tim Barrow, will hand-deliver a letter from May to EU Council President Donald Tusk in Brussels. 
 
 Photos were released of May signing the letter late Tuesday in the Cabinet room at 10 Downing St., under a portrait of Britain's first prime minister, Robert Walpole. 
 
 The letter, which is several pages long, was whisked to Brussels aboard a Eurostar train, British media reported. Barrow arrived at European Council headquarters carrying a briefcase Wednesday morning, before his appointment with Tusk. 
 
 May's office said she will tell lawmakers that the U.K. is embarking on a "momentous journey" and should unite to forge a "global Britain." 
 
 "It is my fierce determination to get the right deal for every single person in this country," she will say. 
 
 Britain's Treasury chief, Philip Hammond, said that triggering Brexit was "a pivotal moment for Britain," but denied the country was taking a leap in the dark. 
 
 "We all have the same agenda. We are all seeking to get the best possible deal for Britain," he told the BBC. 
 
 Hammond said he was optimistic of forging "a relationship that will strengthen the U.K. and will strengthen the European Union as well." 
 
 Gus O'Donnell, the U.K.'s former top civil servant, was less certain. 
 
 "We are in a plane being flown by members of the EU and we're about to jump out and we've got a parachute that was designed by the people flying the plane and they designed it in a way to deter anybody else jumping out," he said. 
 
 Britain and the EU have two years to unpick a tapestry of rules, regulations and agreements stitched over more than four decades since Britain joined what was then the European Economic Community in 1973. 
 
 EU officials are due to circulate draft negotiating guidelines within days, and bloc leaders — minus May — will meet April 29 to adopt a common position. 
 
 Britain says it's not turning its back on its neighbors and wants to remain friends. May has said that the U.K. will become "stronger, fairer, more united and more outward-looking" and will seek "a new, deep and special partnership with the European Union." 
 
 But many British businesses fear the impact of leaving the EU's vast single market of some 500 million people. Senior British officials say they are confident of striking a close new free-trade relationship with the bloc — but a successful outcome to the complex and emotionally fraught negotiations is far from certain. 
 
 Brexit has profound implications for Britain's economy, society and even unity. The divisive decision to leave the EU has given new impetus to the drive for Scottish independence, and undermined the foundations of Northern Ireland's peace settlement. 
 
 It's also a major blow to the EU, after decades of expansion, to lose one of its largest members. Anti-EU populists including French far-right leader Marine Le Pen hope the impulses that drove Britain to turn its back on the EU will be repeated across the continent. 
 
 ___ 
 
 Associated Press writer Danica Kirka contributed to this story.
– The UK is preparing to say adios, au revoir, and auf wiedersehen to the European Union after 44 years of membership. Prime Minister Theresa May signed a letter Tuesday night invoking Article 50, and the EU received it Wednesday, formally triggering two years of negotiations leading up to Britain's departure from the bloc, the BBC reports. May was expected to address British lawmakers, calling the historic moment a "time for the country to come together" and promising to fight for "the right deal for every person in this country." Pro-Brexit British newspapers including the Sun, which beamed the message "Dover and Out" onto the famous white cliffs, are rejoicing, though others have sounded notes of caution, the AP reports. "We are in a plane being flown by members of the EU and we're about to jump out and we've got a parachute that was designed by the people flying the plane and they designed it in a way to deter anybody else jumping out," says former top civil servant Gus O'Donnell. EU budget commissioner Guenther Oettinger described the letter as "a negative message for Europe as a whole, for the UK especially" and predicted months of difficult negotiations ahead.