Declaring there was no turning back, Prime Minister Theresa May formally began Britain's divorce from the European Union on Wednesday.

In one of the most significant steps by a British leader since World War Two, May notified EU Council President Donald Tusk in a hand-delivered letter that Britain would quit the club it joined in 1973.

This ushered in a tortuous exit process that will test the bloc's cohesion and pitch her country into the unknown, said a Reuters report.

"The United Kingdom is leaving the European Union," May told parliament nine months after Britain shocked investors and world leaders by unexpectedly voting to quit the bloc. "This is an historic moment from which there can be no turning back."

The prime minister, an initial opponent of Brexit who won the top job in the political turmoil that followed the referendum vote, now has two years to negotiate the terms of the divorce before it comes into effect in late March 2019.

May, 60, has one of the toughest jobs of any recent British prime minister: holding Britain together in the face of renewed Scottish independence demands, while conducting arduous talks with 27 other EU states on finance, trade, security and other complex issues.

The outcome of the negotiations will shape the future of Britain's $2.6 trillion economy, the world's fifth biggest, and determine whether London can keep its place as one of the top two global financial centres.

For the EU, already reeling from successive crises over debt and refugees, the loss of Britain is the biggest blow yet to 60 years of efforts to forge European unity in the wake of two world wars.

BREXIT DEAL?
May's notice of the UK's intention to leave the bloc under Article 50 of the EU's Lisbon Treaty was hand-delivered to Tusk in Brussels by Tim Barrow, Britain's permanent representative to the EU, on the top floor of the new Europa Building in Brussels.

That moment formally set the clock ticking on Britain's two-year exit process. Sterling, which has lost 25 cents against the dollar since the June 23 referendum, jumped to $1.25.

May signed the six-page Brexit letter on Tuesday night, pictured alone at the cabinet table beneath a clock, a British flag and an oil painting of Britain's first prime minister, Robert Walpole.