By Jenny Gross 

LONDON--The U.K. on Wednesday is set to make official its planned exit from the European Union, starting on an unprecedented path to reshape its relationship with its closest allies in some of the most complex negotiations the country has ever undertaken.

Nine months after Britain voted to leave the EU, Tim Barrow, Britain's ambassador to the bloc, will hand deliver a letter to European Council President Donald Tusk formally notifying the bloc that the U.K. will be the first country ever to leave.

The triggering of Article 50 of the Lisbon Treaty, which governs EU law, will open a two-year window for Britain to negotiate the terms of its exit from the bloc, unraveling 44 years of ties. U.K. government officials say there is no going back from there.

"It is my fierce determination to get the right deal for every single person in this country," Prime Minister Theresa May will say in a speech to Parliament after the letter is delivered, according to excerpts from her office.

The negotiations are expected to be tough and test the unity of the bloc. Early indications are that Britain and the EU are far apart. Mrs. May has said Britain won't abide by the bloc's court or immigration rules once it leaves, but that it wants to continue to trade as freely as possible with the EU. European leaders say they don't want to punish the U.K., but won't grant Britain a better deal outside the club than it has in it.

With only a small majority in Parliament, Mrs. May faces pressure from lawmakers within her party, many of whom are pushing for a complete break with the EU. She must balance these demands without alienating those who voted to stay. In Scotland, where the vast majority of the public wanted to remain, leaders are laying the groundwork for another independence referendum they want to be held once the Brexit terms are known.

Mrs. May had telephone conversations with Mr. Tusk, German Chancellor Angela Merkel and European Commission President Jean-Claude Juncker on Tuesday, a Downing Street spokeswoman said.

They agreed "that a strong EU was in everyone's interests and that the U.K. would remain a close and committed ally," she said. "They also agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process."

One of the first--and likely most contentious--issues in the negotiations will be how much the U.K. will have to pay to exit from the bloc, said Iain Begg, a research fellow at the London School of Economics. While some U.K. officials have said the country shouldn't have to pay any sum, EU officials say the bill, which would include payments for programs the U.K. had committed to before it voted to leave, could be as high as EUR50 billion ($54 billion) to EUR60 billion.

"Failure to settle this thorny, if ultimately straightforward issue quickly could have a damaging effect on other elements of the negotiations," Mr. Begg said.

Mrs. May wants to negotiate the U.K.'s new trading and financial relationship with the 27 other EU member states, during which she will strive to protect London's status as a financial hub. But whether she can accomplish that in the tight two-year time span is unclear.

Mrs. May has said one of her priorities is to guarantee EU citizens living in Britain and U.K. citizens living in other parts of Europe continue to have the right to stay even after the U.K. leaves. The EU has said talks on that issue alone will take at least several months.

The U.K. and the EU will also have to decide what to do with unspent EU funds that were supposed to go to U.K. farmers, how to implement cross-border security arrangements including access to EU security databases, and whether the U.K. will stay under the jurisdiction of EU regulatory agencies for issues, like nuclear power, that are handled by the bloc's institutions.

On Thursday, Mrs. May will publish details of her plans to transfer EU law into U.K. law, so that 19,000 laws and regulations formed over the past four decades will continue to apply to the U.K. after it leaves the bloc, while allowing Parliament to amend the laws as it deems necessary.

Within 48 hours of Britain triggering Article 50, Mr. Tusk will send draft guidelines to the 27 remaining capitals framing the talks from the EU's side. Those guidelines should be finally agreed upon at a Brussels summit on April 29. After that, EU governments and the European Commission, which will lead day-to-day negotiations, will take several weeks to set a detailed negotiating mandate for Michel Barnier, the EU's Brexit negotiator. Only then, possibly in late May, will negotiations begin in earnest.

A key question will be the degree to which EU countries--some with competing interests in negotiations--can stay united.

Laurence Norman in Brussels contributed to this article.

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

(END) Dow Jones Newswires

March 29, 2017 02:55 ET (06:55 GMT)

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