The UK's chief negotiator Olly Robbins will quit his Brexit role this summer.
It is understood the man who masterminded Theresa May's Brexit deal will step down shortly after the new Prime Minister takes up post at the end of July.
Mrs May will step down following her final PM's Questions on July 24 and Mr Robbins is expected to follow shortly afterwards, although he has committed to a brief handover period before he moves on, with reports he may take up a job in the City.
Mr Robbins's departure is the latest in a wave of top civil servants dealing with Brexit who have announced they will resign rather than take on the challenge of delivering the exit within 100 days under the new leader.
Tom Shinner, director of policy and delivery coordination at the Department for Exiting the EU, and Karen Wheeler, director general of Her Majesty's Revenue & Customs Brexit border delivery group, have both announced they will quit.