By Jenny Gross

LONDON--Britain will notify the European Union on March 29 that it will start the process of extricating itself from the bloc, opening a two-year window for negotiations, U.K. Prime Minister Theresa May's spokesman said Monday.

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 next Wednesday, Mrs. May's spokesman said.

"We are on the threshold of the most important negotiation for this country for a generation," David Davis, Britain's Brexit minister, said in a statement. "The government is clear in its aims: a deal that works for every nation and regionof 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 to the European Council will mark a key moment in Britain's path out of the EU, setting in motion the unwinding of 40 years of ties and paving the way for Britain to leave the bloc by March 2019. The negotiations will be some of the most complex either side has undertaken, and the two sides publicly remain far apart on central issues.

Margaritis Schinas, chief spokesman for the European Commission, said the EU's executive was informed by U.K. officials of the decision but it is now waiting for the formal letter of notification. Downing Street didn't say whether the letter would detail Britain's negotiating positions.

EU officials say it will be weeks before negotiations can start in earnest. First, a summit meeting of the leaders of the 27 other EU states will be held, probably in the second half of April, to lay down principles for negotiators. Then, officials from the 27 remaining EU countries will meet to hash out negotiating positions in more detail. The first serious face-to-face talks will take place in late May or June at the earliest, officials say.

Mr. Tusk, president of the European Commission, said this month that he would do everything he could to make sure the EU and the U.K. remained close partners in the future. He said he would set out a response to the Article 50 letter by March 31.

"It is our wish to make this process constructive, and conducted in an orderly manner," Mr. Tusk said.

In June, Britain voted to leave the EU. Mrs. May, who took office after the vote, has said the U.K. wants a clear break from the bloc, leaving the single market for goods and services in order 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.

Mrs. May, whose Conservative Party holds only 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 in Brussels contributed to this article.

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

(END) Dow Jones Newswires

March 20, 2017 09:45 ET (13:45 GMT)

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