By Jason Douglas in London and Valentina Pop in Brussels

LONDON--The U.K.'s ambassador to the European Union resigned Tuesday, in a potential complication for Prime Minister Theresa May as she prepares to trigger talks with the bloc over Britain's exit.

Ivan Rogers had been due to stand down in October, but decided to leave early to allow for the appointment of a successor before divorce talks begin, a U.K. government spokesman said in a statement. He is now due to leave in January. He couldn't be reached for comment Tuesday.

Mrs. May has said she would give the EU formal notice of the U.K's withdrawal from the bloc before the end of March, opening the door to two years of negotiations.

Anexperienced negotiator steeped in the workings of the EU, Mr. Rogers had been expected to play a critical role in Britain's exit. His resignation will deprive Mrs. May of considerable expertise as she heads into talks, said Charles Grant, director of the Centre for European Reform, a London-based think tank that is broadly pro-EU but advocates reform, and makes Mrs. May's task of securing a good deal for Britain "significantly harder."

Downing Street declined to comment.

Just over two weeks ago, the BBC reported that Mr. Rogers had warned U.K. ministers that a Brexit deal could take a decade to strike and could be rejected by other EU parliaments, a stance that drew the ire of pro-Brexit campaigners who accused him of being too pessimistic.

Arron Banks, chairman of pro-Brexit pressure group, Leave.EU, said Tuesday that Mr. Rogers' resignation cleared the way for the appointment of an ambassador "who is optimistic about the future that lies ahead for Brexit Britain."

Mrs. May said in December that she planned to set out more details of the government's Brexit strategy early in 2017, amid pressure for greater clarity.

The U.K.'s departure would be the first time a member state has left the EU and the terms of its departure--as well as its future ties to the bloc--are unclear.

Mrs. May has said she sees the U.K. negotiating a new trade deal with the EU in parallel with negotiations on its divorce terms from the bloc, which some analysts say could be ambitious in a two-year time frame--although talks could be extended. She has also said that a transitional deal could be reached to reduce uncertainty for business.

Sent to Brussels as the U.K.'s permanent representative to the EU by Mrs. May's predecessor, David Cameron, Mr. Rogers previously held senior posts at the U.K. Treasury and served as a top adviser to former Prime Minister Tony Blair.In Brussels, Mr. Rogers was seen as an influential and well-respected diplomat, who played a pivotal role in 2014 and 2015 when the Cameron government negotiated a deal on several key areas in the run-up to the Brexit referendum.

His departure follows that of his deputy, Shan Morgan, who announced in November she would join the Welsh government.

A spokesman for the European Commission, the EU executive body that will lead the Brexit negotiations, declined to comment.

Write to Jason Douglas at jason.douglas@wsj.com and Valentina Pop at valentina.pop@wsj.com

(END) Dow Jones Newswires

January 03, 2017 11:31 ET (16:31 GMT)

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