The European Union flag flies in front of London’s Big Ben on Feb. 20. (Justin Tallis/Agence France-Presse via Getty Images)

Britain’s government said Monday that it will deliver a letter to the European Union next week giving formal notice of its plans to leave the bloc, a widely anticipated step that makes good on last year’s Brexit vote. 

The triggering of Article 50, the never-before-used mechanism for a country to leave the European Union, will set off a two-year negotiation in which the United Kingdom and its 27 erstwhile partners will have to agree on the terms of divorce.

“We are on the threshold of the most important negotiation for this country for a generation,” said David Davis, Britain’s Brexit secretary. “The government is clear in its aims: a deal that works for every nation and region of the U.K. and indeed for all of Europe — a new, positive partnership between the U.K. and our friends and allies in the European Union.”

Prime Minister Theresa May has vowed for months that the country will trigger Article 50 by the end of March. But Monday’s announcement of the date — March 29 — was the first official confirmation of the exact timing.

Britain’s Parliament gave its final approval last week to May’s Brexit plans, and the prime minister had at one point been expected to trigger Article 50 then. 

Scottish leader Nicola Sturgeon threw a wrench into those plans last Monday by announcing a push for a new referendum on independence from the United Kingdom, which also includes England, Wales and Northern Ireland. The Scottish move seemed to catch Downing Street off guard, and it may have contributed to a decision to push Article 50 notification back to the final week of March.

May heads into the E.U. negotiations with her premiership, Britain’s economy and even the United Kingdom’s viability as a unified country all on the line. She came to power soon after the Brexit referendum in June and has repeatedly said that she will deliver on voters’ narrow decision to make Britain the first country to leave the E.U.

On Monday, she departed on the first stage of a “listening tour” that will take her across Britain in the lead-up to the March 29 move. Her first stop was Wales, and she was expected to visit sites in Scotland, Northern Ireland and England in the coming days. 

Although Britain as a whole voted 52 to 48 percent in favor of leaving, majorities in both Scotland and Northern Ireland favored staying in the E.U. Sturgeon has charged that Scottish voters are being taken out of the bloc against their will, and she said last week that she wants a referendum on independence — a rerun of a September 2014 vote, in which a majority of Scottish voters opted to stay in the United Kingdom — between the autumn of 2018 and the spring of 2019. 

May has sharply criticized that call. She said over the weekend that “now is not the time” for a Scottish vote. But she has not threatened to veto another referendum.

Britain’s exit negotiations are expected to be exceptionally tricky, with the country aiming to leave Europe’s single market and customs union but hoping to retain preferential access to both through a new trade agreement. 

May has signaled that she will prioritize Britain’s ability to control immigration from E.U. countries, a critical element driving pro-Brexit sentiment. European leaders have drawn a tough line, signaling that they will not allow Britain to enjoy the benefits of E.U. membership but not bear the responsibilities. 

Once Britain has delivered its Article 50 letter to European Council President Donald Tusk in Brussels, E.U. leaders are expected to reply with a letter setting out the bloc’s negotiating stance.

If Britain and the rest of the E.U. cannot agree to terms by the spring of 2019, they will have to extend the negotiations or Britain will simply fall out of the bloc without an agreement on its future relations with its biggest trading partner — a scenario known as “dirty Brexit.” 

May is hoping that she will be able to run for reelection in the spring of 2020 on a platform of having delivered on the public’s will. But economists and government officials have warned that Britain’s exit is likely to be turbulent, and some within the prime minister’s ruling Conservative Party have urged her to call for an early election this spring. 

The call would take advantage of polls showing May’s Tories well ahead of the opposition Labour Party, which has been beset by internal strife under left-wing leader Jeremy Corbyn. May has a narrow majority in the House of Commons, and a vote this spring probably would allow her to broaden it significantly.

But she has repeatedly ruled out an early vote, and her spokesman told British journalists on Monday that there is “not going to be one.”