British Prime Minister Theresa May has started the formal process that will divorce the United Kingdom from the European Union, overturning four decades of integration with its neighbors and shaking the foundations of a block 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," May told lawmakers in the House of Commons. “This is a historic moment from which there can be no turning back. The United Kingdom is leaving the European Union.”

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She added: “We are leaving the European Union but we are not leaving Europe.”

Tusk tweeted that "after nine months the UK 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 said during a speech later, emphasizing that the priority now is to minimize costs for EU citizens and member states.

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.

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 was 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."

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May's six-page letter to Tusk triggering Article 50 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."

May said it is in the "best interests of both the United Kingdom and the European Union that we should use the forthcoming process to deliver these objectives in a fair and orderly manner, and with as little disruption as possible on each side."

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.

European leaders expressed dismay, with Danish Prime Minister Lars Loekke Rasmussen saying Britain's goodbye to the European Union is "incredibly sad" and he expects "many bumps on the road."

Germany's Foreign Ministry said Britain would remain a "close partner and friend" of the EU, but that "being a close friend is not the same as being part of the family."

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 it's 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."

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.

May said Britain's aim was to reach a deal on divorce terms and a new relationship within the two years -- something EU officials say is unlikely. May conceded Wednesday that there would have to be a "phased process of implementation."

Brexit Secretary David Davis — the man charged with leading Britain's side in the talks — has called it "the most complicated negotiation in modern times, maybe the most complicated negotiation of all time."

Tusk has said that within 48 hours he will respond with a draft negotiating guidelines for the remaining 27 member states to consider. Leaders of those nations will then meet on April 29 to finalize their negotiating platform before instructing the EU's chief negotiator, French diplomat Michel Barnier.

Then Barnier will sit down with his British counterpart, Davis, who has said the first item on the agenda will probably be: "How we do this?"

The Associated Press contributed to this report.