Boris Johnson has been warned he faces "constitutional crisis" over his "do or die" Brexit plans within weeks of entering Downing Street, ahead of his expected confirmation as Britain's next prime minister on Tuesday.
His anticipated election by an estimated 160,000 Conservative party members will be followed by a government walk-out, with further pro-EU ministers expected to follow chancellor Philip Hammond, justice secretary David Gauke and international development secretary Rory Stewart in resigning before they can be sacked.
And Sir John Major became the third former PM in as many days to issue a stern warning about a Johnson premiership.
Whoever succeeds Theresa May as prime minister “must choose whether to be the spokesman for an ultra-Brexit faction or the servant of the nation he leads,” said the former Tory premier. “He cannot be both, and the choice he makes will define his Premiership from the moment of its birth”.
Mr Johnson narrowly avoided being blocked at the last minute from fulfilling his lifetime ambition of becoming PM.
His former deputy at the Foreign Office, Sir Alan Duncan, quit the government in order to table a motion in the Commons to test whether the new Tory leader could command a majority of MPs.
The motion was rejected by Speaker John Bercow, but if it had been allowed to go to a vote on Wednesday, it might have prevented Ms May from recommending to the Queen that Johnson was the person best able to form a government after her departure.
Unless his rival Jeremy Hunt pulls off one of the most remarkable political turnarounds of modern times, Mr Johnson is expected to be confirmed as new Tory leader around noon on Tuesday.
But Ms May will remain in office for one more day and face her final session of prime minister’s questions in the Commons on Wednesday before going to Buckingham Palace to offer her resignation to the Queen. Moments later, her successor - almost certainly Mr Johnson - will follow her into the Palace to “kiss hands” with the monarch.
The former foreign secretary will move quickly to appoint a cabinet to take forward his promise to take the UK out of the EU by the end of October, deal or no deal, in a parliament where he commands a working majority of just four - expected to be reduced to three by next week’s by-election in Brecon and Radnorshire.
He is likely to issue an appeal for unity in a Conservative party which has become mired in vicious infighting over Europe under Ms May’s leadership.
Former ministers warned that he risks plunging the party into further turmoil unless he reflects the range of opinion on Brexit in his top team.
“He has been surrounded by members of the parliamentary party who have extreme views about the appropriateness of a no-deal Brexit,” said Guto Bebb, who quit the government over Brexit last year. “I’m not convinced he can pivot back to the centre, and that is going to bring the possibility of a no-deal outcome closer.”
The Aberconwy MP told The Independent it was a “fallacy” to believe there was a majority to be won in the Commons or the country for no-deal Brexit. But he said last week’s victory for a motion preventing Johnson from suspending parliament to force no-deal through meant he was unlikely to face a no-confidence vote before the autumn.
“People are taking the view that he can be given the summer to travel around, giving the impression of activity while not necessarily achieving very much,” he said. “But I think we will be in a pretty severe constitutional crisis within six to eight weeks.”
Former culture minister Margot James, who walked out of the government last week over Mr Johnson’s threat to suspend parliament, said she expected Tory MPs to give Mr Johnson an opportunity to show he can negotiate the acceptable withdrawal deal that eluded Ms May.
“A lot of us will do all we can to support him in a genuine attempt to get a deal that will pass through parliament and be acceptable to the EU and Ireland,” she said.
“If he can do that, he will have my support. But I’m certainly not prepared to catapult out of the EU without a deal, which would be completely contrary to some of our manifesto commitments and a lot of what was said in the referendum as well.”
Steve Brine, who quit as a health minister in March to vote against the government on Brexit, said Johnson’s future may depend on whether he governs as “good Boris” or “bad Boris”.
“He will start with a clean slate as far as I’m concerned,” said Hunt supporter Mr Brine, who stressed he had not given up hope of victory for the foreign secretary. “If he appears to be a unifier and reaches across different sections of the parliamentary party, then I think people will be willing to give him a chance. A lot depends on who he appoints to senior cabinet positions - that will show how conciliatory he intends to be.”
Prominent Brexiteer Jacob Rees-Mogg said Mr Johnson was the right person to make the Tories “electable” and said he would be ready to serve in any position in his administration.
“I think he's the right answer, I think he'll be a very good Prime Minister,” the chair of the influential European Research Group told LBC. “Charisma is the stardust of politics. It's difficult to define. It's difficult to pin down. Boris just has it and other people - almost everyone else - don't have it.
“I think he will have the right spirit in getting us out of the European Union and moving onto the other things that we need to do. So if he wants me to be the junior under-minister in Pyongyang, I will happily go off and do it.”
Sir Alan, who served for two years under Mr Johnson at the Foreign Office, made clear he would not serve under a man he has described as a “circus act” and said it was tragic that the UK’s influence in the world was being undermined by ”the dark cloud of Brexit”.
He insisted his actions were not motivated by "personal animosity of any sort" towards Mr Johnson but told the BBC: “I have very grave concerns that he flies by the seat of his pants and it's all a bit haphazard and ramshackle".
Mr Hunt said Duncan would be "greatly missed" at the Foreign Office, but hinted at a swift return to government for Sir Alan if he wins the race to succeed Ms May, telling MPs his absence would be "not for long, if the results go the way I am hoping".