LONDON (AP) - The United Kingdom filed for divorce from the European Union on Wednesday, overturning four decades of integration with its neighbors, demolishing the notion that EU expansion is inevitable and shaking the foundations of a bloc that is facing challenges to its identity and its place in the world.
Britain's top envoy to the EU, Tim Barrow, hand-delivered a letter to European Council President Donald Tusk formally triggering a two-year countdown to the final split.
"Today the government acts on the democratic will of the British people," Prime Minister Theresa May told lawmakers in the House of Commons, adding, "This is an historic moment from which there can be no turning back."
Tusk tweeted that "after nine months the U.K. has delivered," followed by a photo of Barrow handing him the letter in front of British and EU flags in Brussels.
There is "no reason to pretend this is a happy day," Tusk told reporters later, emphasizing that the priority now is to minimize costs for EU citizens and member states.
To Britain, he said: "We already miss you."
But for Britons who voted 52 to 48 percent to leave the bloc in a referendum nine months ago, it was a time for celebration.
"I voted for Brexit and today is the day that vote starts to count," said Charles Goodacre, a former taxi driver, in the northern England city of Sunderland. "Things have been bad round here for a while and we needed a change. There's been a lot of arguments about what happened but we can now get on with it."
Former U.K. Independence Party leader Nigel Farage, who campaigned for years to take Brexit from fringe cause to reality, said Britain had passed "the point of no return."
"I can still, to be honest with you, scarcely believe today has come," he said.
For "remain" campaigners, it is now time to fight for a divorce settlement that preserves what they see as key benefits of EU membership, including free trade in goods and services and the right to live and work anywhere in the bloc.
"The phony war is over," said Joe Carberry, co-director of the pro-EU pressure group Open Britain. He said Britain had decided that it would leave the bloc - but "the issue of how we will leave, and the democratic checks and balances along the process of the negotiations, remains unresolved."
May's six-page letter to Tusk was polite and conciliatory, stressing that Britons want to remain "committed partners and allies to our friends across the continent."
She said the two sides should "engage with one another constructively and respectfully, in a spirit of sincere cooperation."
But there was a hint of steel in May's assertion that without a good deal, "our cooperation in the fight against crime and terrorism would be weakened." That could be seen by some in Europe as a threat to withdraw British cooperation on crime and counterterrorism if the U.K. does not get its way.
The loss of a major member is destabilizing for the EU, which is battling to contain a tide of nationalist and populist sentiment and faces unprecedented antipathy from the new resident of the White House.
It is even more tumultuous for Britain. For all the U.K. government's confident talk of forging a close and friendly new relationship with its neighbors, it cannot be sure what its future relationship with the bloc will look like - whether businesses will freely be able to trade, students to study abroad or pensioners to retire with ease in other EU states. Those things have become part of life since the U.K. joined what was then called the European Economic Community in 1973.
It's not even certain that the United Kingdom will survive the exit intact. Scotland's parliament voted Tuesday to back First Minister Nicola Sturgeon's call for a referendum on independence within two years. Scottish voters backed remaining in the EU in last year's vote, and Sturgeon insists Scotland must not be "taken down a path that we do not want to go down without a choice."
May insists "now is not the time" for a referendum, setting her on course for a showdown with the Edinburgh administration just when the U.K. government wants to devote all its energies to the EU talks.
The trigger for all the economic and constitutional uncertainty is Article 50, a previously obscure clause of the EU's Lisbon Treaty that allows a member state to withdraw from the bloc. 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 fifth-largest economy, and the EU, a vast single market stretching over 27 countries and half a billion people.
Tusk said he will respond by Friday with draft negotiating guidelines for the remaining 27 member states to consider. Leaders of those nations will meet April 29 to finalize their negotiating platform before instructing the EU's chief negotiator, French diplomat Michel Barnier.
Talks between Barnier and his British counterpart, Brexit Secretary David Davis, are likely to start in the second half of May. Barnier on Wednesday tweeted a group photo with the words "Our #Brexit team is ready. We will work for #EU27 member states, EU institutions & citizens; together with all Commission services."
As in many divorces, the first area of conflict is likely to be money. The EU wants Britain to pay a hefty bill - Jean-Claude Juncker, president of the EU's executive Commission, put it at around 50 billion euros ($63 billion) - to cover pension liabilities for EU staff and other commitments the U.K. has agreed to.
Britain acknowledges it will have to pay something, but is sure to quibble over the size of the tab.
Negotiations will also soon hit a major contraction: Britain wants "frictionless" free trade, but says it will restore control of immigration, ending the right of EU citizens to live and work in Britain. The EU says Britain can't have full access to the single market if it doesn't accept free movement, one of the bloc's key principles.
Both Britain and the EU say a top priority will be guaranteeing the rights of 3 million EU citizens living in Britain, and 1 million Britons living elsewhere in the bloc.
The two sides also appear to disagree on how the talks will unfold. EU officials say the divorce terms must be settled before negotiators can turn to the U.K.'s future relationship with the bloc - and a deal on that could take a decade. British officials want the two things discussed simultaneously.
May conceded Wednesday that there would have to be a "phased process of implementation" once the two years are up.
May has suggested that if talks stall she could walk away, saying that "no deal for Britain is better than a bad deal for Britain." That prospect of no deal alarms many British businesses. If Britain crashed out of the EU without a trade deal it would fall back onto World Trade Organization rules, meaning tariffs and other barriers to trade.
May backed away from that notion Wednesday, saying in her letter that both sides should "work hard to avoid that outcome."
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.
Many Britons who voted to leave were seeking to regain control of migration, by removing the U.K. from the EU's principle of free movement among member states.
That is what worries many on the other side. Many British businesses rely on European workers, and about 3 million citizens of other EU states live in the U.K. - and have been uncertain about their future since the referendum in June.
Piotr Wierzbicki, a Polish engineer flying to London from Warsaw airport, said that the British "shot themselves in the foot" by voting to leave the EU, and predicted it would be harder for Europeans to travel to and from the U.K.
"It will be bad for their economy and it will be bad for the EU," he said
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Follow Jill Lawless on Twitter at http://Twitter.com/JillLawless
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Associated Press journalists Danica Kirka, Siobhan Starrs and Jonathan Shenfield in London, Raf Casert and Lorne Cook in Brussels, Jan M. Olsen in Copenhagen and Monika Scislowska in Warsaw contributed to this story.