By Jenny Gross 

LONDON -- Britain on March 29 will formally trigger negotiations to remove itself from the European Union, opening a two-year window for talks set to disentangle decades of close ties and redefine Britain's relationship with some of its closest allies.

Britain's ambassador to the EU, Tim Barrow, told the office of European Council President Donald Tusk on Monday morning that Britain would trigger Article 50 of the Lisbon Treaty, the formal withdrawal mechanism, a week from Wednesday, said James Slack, a spokesman for Prime Minister Theresa May.

Mrs. May has said she would trigger the U.K.'s exit by the end of this month, but the exact date had been left open amid months of wrangling in Parliament and the courts. Negotiations are now likely to begin in earnest in early summer.

"We are on the threshold of the most important negotiation for this country for a generation," said David Davis, Britain's Brexit minister. "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."

Mrs. May's letter will pave the way for Britain to leave by March 2019, putting her at the center of EU politics as antiestablishment and euroskeptic movements challenge the bloc. EU leaders are grappling with whether the bloc should continue its deep political and economic integration or put a brake on broader ambitions for unity.

Lawmakers and EU negotiators will be watching Mrs. May's speech to Parliament next week for further clues about the approach Britain will take when it goes to the bargaining table and for indications of how flexible Mrs. May's team may be.

The negotiations will be some of the most complex either side has undertaken, and the two sides publicly remain far apart on some central issues. Downing Street didn't say whether the letter would give more details on Britain's negotiating positions.

British voters decided to leave the bloc in June, but the country's Supreme Court ruled in January that Mrs. May needed parliamentary approval to trigger Article 50, casting doubt on her timeline. She got the go-ahead from lawmakers last week.

"Finally, finally the negotiations can begin," said French Finance Minister Michel Sapin. "After the Brexit vote, which we have to respect, it took quite some time. I believe the U.K. needed some time to prepare, but finally we can enter negotiations and I hope we can do it constructively on both sides."

Mr. Tusk said he would set out a response to the Article 50 letter by March 31.

EU officials said the late-March trigger would delay the start of real negotiations between the U.K. and the rest of the EU, meaning they may not begin until early summer. That is because there is now too little time to convene the 27 other EU heads of government for a meeting in early April as Mr. Tusk had originally planned, they said.

That meeting will be key as it would settle the guidelines for the talks -- setting out which issues will be dealt with in the divorce negotiations and in what order. After that, the EU will need another few weeks to turn those guidelines into a formal negotiating mandate for Michel Barnier, who will lead the day-to-day talks for the bloc.

An EU official said no specific date had been set for the meeting of the bloc's other leaders. "But we expect to need approximately four to six weeks to prepare and consult with EU 27 member states."

Mrs. May, who took office after the Brexit vote, has said the U.K. wants a clear break from the bloc, leaving the single market for goods and services to take control over immigration from the EU. But she says she wants the best possible trade deal with the EU that the U.K. can get.

EU officials say Britain owes it EUR55 billion ($59 billion) to EUR60 billion to cover budget commitments already made, future pension liabilities and other items. When Mrs. May's government published a government report outlining its objectives, it skirted over the exit bill and other key issues, which are expected to dominate negotiations over Britain's divorce terms.

EU policy makers have said Britain must recognize it must settle outstanding commitments early on in the talks if discussions are going to move on to address Britain's future trade and economic relationship with the bloc.

Jeroen Dijsselbloem, the chairman of the eurozone finance ministers group, said he hopes to see "realism" in the Article 50 notification. "Realism over the price it will cost, realism over the complexity and the time needed, which so far we have missed quite a lot from the part of the British government. But we will see," he said.

Mrs. May, whose Conservative Party holds a thin majority in Parliament, must tread carefully as she embarks on negotiations. Some in Parliament say her positions are too hard-line. She also faces political pressure from Scotland, where the governing party is calling for a second referendum on independence from the U.K. This has raised the prospect that the U.K. could itself split apart as it is unraveling its ties to the EU. The majority of people in Scotland voted to remain in the EU.

The spokesman said Mrs. May didn't have plans to hold an early general election, addressing rumors that she planned to call one in the coming months to increase her majority in Parliament.

--Laurence Norman and Valentina Pop in Brussels contributed to this article.

Write to Jenny Gross at jenny.gross@wsj.com

(END) Dow Jones Newswires

March 20, 2017 16:12 ET (20:12 GMT)