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 12:30 p.m. local time (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.

Britain will remain an EU member while it negotiates its departure, but Wednesday's letter is a historic — and, Britain says, irreversible — step through the exit door.

"At 12:30 we pass the point of no return," said former U.K. Independence Party leader Nigel Farage, a longtime campaigner for Brexit.

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.

Hammond told the BBC 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.

Hilary Benn, chairman of Parliament's Brexit committee, said last year's referendum "determined we're leaving, but it did not determine the way in which we leave, and the nature of the new relationship with our friends and neighbors in Europe."

He said "there is everything to play for in the negotiations, but it's going to be a very complex task and now is a moment for frankness about the scale of the challenge."

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.

"It will be bad for their economy and it will be bad for the EU," he said.

But David Kerr, a London electrician who voted to leave, said there was no going back.

"I think people have spoken in the Brexit vote," he said. "You have to bow to the will of the people. It's a simple as that."

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Associated Press writers Danica Kirka in London and Monika Scislowska in Warsaw contributed to this story.