(CNN) — Ivan Rogers, Britain’s ambassador to the European Union, has resigned just months before Brexit negotiations are due to begin, the Press Association reported Tuesday.

Before he quit, Rogers was due to lead the UK’s negotiations to leave the EU.

Prime Minister Theresa May has said the UK will begin the formal process of doing so by the end of March.

News of the resignation comes weeks after the EU’s chief Brexit negotiator, Michel Barnier, warned Britain that it may only have 18 months to strike an exit deal from the bloc.

Barnier said the process would need to be completed by October 2018 to allow for ratification by the 27 remaining member states within the two-year time scale prescribed by EU rules.

“Should the UK notify the council by the end of March 2017, as Prime Minister Theresa May said she would, it is safe to say negotiations could start a few weeks later and an Article 50 agreement be reached by October 2018,” he said.

Barnier warned the time available to conclude a deal would be short. “All in all, there will be less than 18 months to negotiate.”

Rogers, who had been in his post since November 2013, reportedly told ministers in October that a new free trade agreement with the EU could take 10 years to negotiate. The prime minister’s office said that did not reflect the government’s view.

News of his departure was unexpected, with some speculating that it will damage Britain’s negotiating position.

Charles Grant, director of the Centre for European Reform, said in a tweet that Rogers was one of very few people at the top of the British government who understand the EU, and said his resignation “makes a good deal on Brexit less likely.”

Paul Brannen, North East of England Member of the European Parliament, said it was “a huge loss,” adding that “without him to guide them ministers will flounder badly.”