Tory leadership contenders who promise to take on the EU and re-open the Withdrawal Agreement if they take over from Theresa May will "wreck any prospect" of getting a future trade deal, according to Britain's former ambassador to Brussels.
Sir Ivan Rogers, who stood down from his role in January 2017, said he was a "little bit surprised" the UK was not further down the exit process as he spoke to BBC Two's Newsnight programme.
He said the public still do not know what the Prime Minister thinks, and would not say if he would vote for her deal if he was an MP, saying it was "difficult to see a path through" the current negotiations to a short-term solution.
The 59-year-old ex-civil servant also said the Government must "bring the country behind one version of Brexit in the next two to three years - otherwise we're going to re-fight this civil war for the next generation".
Asked about why he left his role eight months earlier than when his posting was due to finish in October, he said it would be wrong to "change horse in mid-stream", and he needed to go before the talks started once Article 50 was triggered in March 2017, or stay on right until the final deal was agreed with the EU.
Sir Ivan said he was "very happy" to stay on and "deliver the best possible Brexit", adding: "But you do have to feel you've got a receptive audience and I didn't really feel by the Christmas of 2016 that was the case."
The former ambassador was asked if he knows what the PM thinks, replying "we all don't know".
On who should succeed her as Conservative leader, he said the party will likely want a "true believer" to take over, saying: "If various candidates make pledges as to the future direction of the Brexit talks, what they would do in phase two, that will essentially wreck any prospect of phase two succeeding.
"So if people were to give commitments, saying 'when I'm in power, if you give me this job, I will reopen the Withdrawal Agreement', and indicate we can't possibly accept the backstop and take a much more robust and bellicose position with Brussels - well, that leads fairly inexorably to a breakdown of the talks."
Asked if he would vote for Mrs May's deal if he was an MP, Sir Ivan replied: "That's an almost impossible question and I'm glad I don't have to answer it in practice.
"The reason in the end that I would vote for a Withdrawal Agreement is I don't believe that the alternative of going to no deal is going to be better for the country."
Having been accused by Brexiteers of being pessimistic about leaving the EU, he said he was "probably not pessimistic enough in 2016 about where we were going to reach by now", and said in the short-term: "I find it quite difficult to see a path through to how this ends easily."
But he said over the long-term, he was "optimistic about the UK".