By Jason Douglas in London and Valentina Pop in Brussels

The U.K.'s ambassador to the European Union resigned, 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 with the EU begin, a U.K. government spokesman said Tuesday.

Mr. Rogers couldn't be reached to comment. But in an email to staff at the British mission in Brussels reported in British media and confirmed by a U.K. official, he appeared to express frustration with the Brexit process, noting that negotiating experience is in "short supply" in the U.K. government.

He expressed hope that his colleagues would "continue to challenge ill-founded arguments and muddled thinking and that you will never be afraid to speak the truth to those in power," according to the reports. "I hope that you will support each other in those difficult moments where you have to deliver messages that are disagreeable to those who need to hear them."

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. An experienced negotiator steeped in the workings of the EU, Mr. Rogers had been expected to play a critical role in Britain's exit.

His resignation makes Mrs. May's task of securing a good deal for Britain significantly harder as she heads into the talks, said Charles Grant, director of the Centre for European Reform, a London-based think tank that is broadly pro-EUbut advocates reform.

Downing Street declined to comment on Mr. Rogers's resignation and his email.

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 reach 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 that Mr. Rogers's 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 talks on its divorce terms from the bloc. Some officials say those goals could be too ambitious for a two-year time frame--although talks could be extended. She has also said that Britain may seek a transitional deal 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 also 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 diplomat. He 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 04, 2017 05:12 ET (10:12 GMT)

Copyright (c) 2017 Dow Jones & Company, Inc.