Chief negotiator Olly Robbins will quit his Brexit role this summer.
PA understands the man who masterminded Theresa May’s Brexit deal will step down shortly after the new prime minister takes up his post at the end of July.
Mr Robbins has committed to a brief handover before he moves on, with reports he may take up a job in the City.
Mr Robbins’ 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 Brexit 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.
Tory leadership contender Jeremy Hunt was quizzed about the report at a hustings in Manchester and said it is “a good moment to change our approach” to cheers from Tory members.
Mr Hunt said: “I didn’t agree with the approach that we took in a number of areas of the Brexit negotiations.
“I was a loyal Cabinet minister – I think it’s very important the Prime Minister has a loyal Foreign Secretary – but I wasn’t a believer in the backstop and I didn’t believe it would get through Parliament, so I think this is a good moment to change our approach.
“I’m not going to say something about a civil servant who I know works incredibly hard, even though there are issues I disagree with him on.
“But I do think now is a moment that we have to get this right, deliver a different deal and one that can actually get through Parliament.”
Both candidates are expected to unveil new Brexit negotiating teams shortly, with Mr Hunt recruiting two Canadians with the aim of securing a Candian-style free trade deal, and Mr Johnson drafting in Brexiteers including Jacob Rees-Mogg.