Sir Ivan Rogers, who stood down from his role in January 2017 and is a vocal critic of Mrs May’s Brexit plan, warned Eurosceptic leadership challengers such as Boris Johnson and Dominic Raab could make promises to the party’s grassroots to get elected but which the EU would never agree to. These could include reopening Theresa May’s withdrawal agreement to change or completely eliminate the Irish backstop, something the EU has insisted it will not do. Sir Ivan warned this type of pledge would destroy negotiations and and chance of London and Brussels agreeing to a new trade relationship.

He told BBC Two's Newsnight programme: “I think the danger that we face and that everyone faces, and I think people are acutely conscious of this in Brussels and Strasbourg and Paris and Berlin, is that if we were to have a Conservative leadership election, might we have a sort of 2019 version of the syndrome I have described in 2016?

“Where in the process of appealing to the party base which is after all more fervently Eurosceptic than many of the parliamentarians and may well want a more true believer Brexiteer as their leader, will various candidates give pledges about what they would do in phase two, that would essentially wreck any prospect of phase two succeeding.”

He added: “For example, if people were to give commitments that ‘if you give me this job I will reopen the Withdrawal Agreement, signal that we can’t possibly accept the backstop and take a more robust and bellicose position with Brussels’ – that leads fairly inexorably I think to a breakdown in the talks.”

Sir Ivan also warned a Brexiteer at the Tory leader would pose a “serious risk” to the negotiating process and that the EU is already considering the implications.

He added: “They will know that a Johnson premiership or a Raab premiership or many other premierships will want to go further out and have a more distant relationship with the EU than Theresa May.

“They will deal with whatever they have to deal with. They may regret that, but they’ll deal with that.

“There may be an appetite from some of the candidates to say ‘to hell with this, I’m going to run this process differently’.

“The difficulty is that the other side has locked itself in into this very intransigent position and says the Withdrawal Agreement is closed.”