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 UK 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, 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 UK 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 UK and will strengthen the European Union as well.”

Gus O'Donnell, the UK'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 neighbours and wants to remain friends. May has said that the UK 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.