UK Prime Minister Theresa May formally began Britain’s divorce from the European Union yesterday, saying there was “no turning back” from a decision pitching her country into the unknown and triggering years of fraught negotiations.

Nine months after Britons voted to leave, May notified EU Council President Donald Tusk in a letter that Britain was quitting the bloc it joined in 1973.

“The United Kingdom is leaving the European Union,” May later told parliament in London. “This is an historic moment from which there can be no turning back.”

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

The 60-year-old 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 US$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 centers.

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.

Its leaders say they do not want to punish Britain. But with nationalist, anti-EU parties on the rise across Europe, they cannot afford to give London generous terms that might encourage other member states to break away.

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.

Barrow gave the letter to Tusk, the EU summit chair and former Polish prime minister, in the council president’s offices on the top floor of the new Europa Building.

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 US$1.25.

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

The letter set a positive tone though it admitted that the task of extracting the UK from the EU was momentous and that reaching agreements within two years would be a challenge.

May wants to negotiate Britain’s divorce and the future trading relationship with the EU within that period, though EU officials say that will be hard.

“We believe it is necessary to agree the terms of our future partnership alongside those of our withdrawal from the EU,” May wrote, adding that London wanted an ambitious free trade agreement with the EU.

“If, however, we leave the European Union without an agreement the default POSTTTION is that we would have to trade on World Trade Organization terms,” she said.

May has promised to seek the greatest possible access to European markets but said Britain was not seeking membership of the “single market” of 500 million people as she understood there could be no “cherry picking” of a free trade area based on unfettered movement of goods, services, capital and people.

May has said Britain will aim to establish its own free trade deals with countries beyond Europe, and limit immigration from the continent.

EU leaders will welcome assurances of a constructive approach and appreciate a commitment to remain a close partner for the EU and to encourage its development, as well as an explicit recognition Britain cannot retain the best bits of membership after leaving.

They may be less warm to an implication that Britain could live with a breakdown of talks on trade coupled with what might be seen as a threat to disrupt the security and counter-terrorism cooperation for which Britain, as a member of the US-backed Anglophone Five Eyes system, is highly valued.

“We should work together to minimize disruption and give as much certainty as possible,” May said. “Weakening our cooperation for the prosperity and protection of our citizens would be a costly mistake.”

Tusk said the EU would seek to minimize the cost of Brexit to EU citizens and businesses and that Brussels wanted an orderly withdrawal for Britain.

“We already miss you,” Tusk said. “Thank you and goodbye.”