Labour MPs are being told they must attend the Commons the day after Theresa May quits ready for the move. Party chairman Brandon Lewis and Chief Whip Julian Smith are said to believe at least two Conservative MPs are ready to defect and others could follow to bring Mr Johnson down. The party has a working majority of just four but an upcoming by-election in Brecon and Radnorshire could see that reduced to three with many tipping the Liberal Democrats to take the seat.
Party chiefs fear that Remainer former ministers Guto Bebb and Phillip Lee are ready to switch party, effectively wiping out the government’s working majority.
Former Attorney General Dominic Grieve said there was a “large number” of Conservatives who would “do everything possible” to stop a no-deal Brexit.
“If the new Prime Minister announces taking the country on a magical mystery tour towards an October 31 crash-out, I don’t think that Prime Minister is going to survive very long,” he added.
Brexiteers are also ready to topple Mr Johnson if he makes it to No 10 but then fails to take Britain out of the EU by October 31.
MPs in the European Research Group of Brexiteer Tories insisted they will force a confidence vote if he does not stick to his promise of leaving in the autumn with or without a deal.
“If Boris reneges on his promises, he’ll be out quicker than May was,” a source told a Sunday newspaper.
A group of senior Tories publicly signalled there would be trouble for the next Prime Minister if they failed to stick to the deadline.
Former Cabinet ministers Esther McVey and Owen Paterson along with ex-Brexit minister David Jones, Tory MP Anne-Marie Trevelyan and the DUP’s Brexit spokesman Sammy Wilson signed to the “Stand Up 4 Brexit” campaign to ensure Britain is out by October 31.
They said Theresa May’s agreement was dead and must be abandoned.
It follows fears among some of Mr Johnson’s supporters that he has softened his language about meeting the exit date.
Former leadership contender Ms McVey said: “Brexit is an opportunity for us to become a truly global country, not a problem to be managed, mitigated and ultimately reversed.
“The only way we will re-connect with voters and bring trust back to our politics is by standing up for Brexit and delivering the historic referendum result by 31st October.”
Mr Jones said: “This needless uncertainty over Brexit must come to an end by 31st October.
“The withdrawal agreement has been defeated by Parliament three times and only one in five voters think that it delivers Brexit. It’s time it was ditched and that we embraced our bright future outside the EU, trading with the fastest growing economies in the world.”
Mr Paterson said: “We must accept that the deal is dead and prepare to leave the EU with a Free Trade Agreement or if necessary on managed World Trade Organisation terms.”
But the next Prime Minister could help push a new deal through with the help of Labour MPs, according to one of the party’s former ministers.
Caroline Flint said the votes of a 26-strong group that want the referendum result to be respected were up for grabs and urged the victor in the leadership contest to “reach out” to secure their support.
She warned the Labour party would become pointless if it abandoned its working class heartlands and backed a second referendum.
The group, made up from Leave and Remain MPs who believe Brexit must be delivered, sent a letter to leader Jeremy Corbyn last week calling for him to rule out a “toxic” second referendum.
“I think those people who signed the letter would like to still have the opportunity to vote on a deal, and don’t forget that when Theresa May was finally forced to stand down we were in discussions about what the withdrawal agreement would be like and changes to the political declaration,” she told BBC One’s Andrew Marr Show.
Mr Corbyn will meet union leaders this week to discuss Labour’s Brexit policy after pressure from senior figures to turn it into a Remain party.
But Ms Flint warned that polling showed all the seats in Sunderland and Barnsley as well as many in Wolverhampton, Stoke and other areas would go.
“One of the polls suggest it’s something like 40 plus,” she said. “We would be losing in our heartland seats.
“And this isn’t just about, you know, electoral advantage; this is actually going to the heart of what the Labour Party is about. If we do not speak for the working class voices of Britain then what’s the point of a Labour Party?”