Theresa May faces "a tremendous amount of political crisis and rupture" if she does not ditch her Chequers plan for leaving the EU, a former Brexit minister has warned.
Steve Baker, who quit the government in July in protest at the prime minister's proposals, is warning Downing Street that around 80 Tory MPs are prepared to vote down the Chequers plan in the House of Commons.
With just 200 days until Brexit, the Wycombe MP also put Mrs May on notice to ditch her Brexit blueprint before the Conservative Party conference later this month, warning of a "catastrophic split" in Tory ranks if she sticks to her current strategy.
A cabinet minister has dismissed Mr Baker's claim that a large number of Tory MPs are prepared to act against the Chequers plan in a parliamentary vote.
But, backing up his declaration that 80 Conservative MPs would side against the prime minister in a vote on her Brexit proposals, Mr Baker highlighted how a similar-sized group of Tory Brexiteers had been ready to vote against the government prior to parliament's summer break.
Ahead of the Taxation (Cross Border Trade) Bill passing through the Commons in July, Tory eurosceptics added amendments to the legislation in an apparent bid to kill off key parts of the Chequers plan.
The Brexiteers were ready to back the changes in a show of force to the government, before Mrs May subsequently accepted their demands and avoided a parliamentary showdown.
Speaking to Sky News' deputy political editor Beth Rigby on Monday, Mr Baker said: "That's why I've said, on the record, that there are those numbers, because people shouldn't be under any illusions that the Conservative Party does face a catastrophic split if this plan is pushed through.
"And, particularly, if it's pushed through with Labour votes.
"About the only thing the Conservative Party has come together on around Chequers is that it is unacceptable, unviable and won't go through."
Prior to his spell in government, Mr Baker led the European Research Group (ERG) of Tory eurosceptics and was also a key critic of the deal David Cameron agreed with the EU prior to the 2016 Brexit vote.
He denied opposition to Mrs May's Brexit deal - based on Brexiteers' fears it leaves the UK too closely aligned to EU rules and will restrict Britain's ability to sign new trade deals - should also necessitate pushing the prime minister out of Downing Street.
Mr Baker said: "I'm not here today to either threaten the prime minister or make any suggestion about who should replace her.
"What we want to do is change the policy not the person... it's not for us to start pushing for any leadership change."
He also appeared to confirm reports Tory Brexiteers have abandoned supposed plans to present their own blueprint for leaving the EU.
Mr Baker added Conservative eurosceptics are merely asking the prime minister to take up the EU's offer of a free-trade agreement.
He said: "We're only asking for what the EU offered us. And what the EU offered us fits within their framework of law and the red lines which determine what a true Brexit would be.
"So, this is not about purity, this is about pragmatism, it's about fitting within the framework of EU law and delivering a Brexit which will work sustainably for the entire UK."
If the prime minister sticks with her Chequers plan beyond this month's Tory conference, Mr Baker is warning that she faces "unchartered territory".
Noting how leading Tory Remainers such as Nick Boles are also unsatisfied with Chequers, he said: "It's inconceivable to me the Conservative Party either in the country or in parliament is capable of uniting around Chequers."
Mrs May's official spokesman insisted on Monday that Chequers was "the only plan on the table which will deliver on the will of the British people while avoiding a hard border in Northern Ireland".
"The prime minister is working hard to secure a deal and hopes all MPs will be able to support it," he added.
Earlier, Justice Secretary David Gauke dismissed Mr Baker's claims 80 MPs would vote down the Chequers plan in parliament.
He told Sky News: "This is a process that's going to require compromises from all sides and I think it's really important that we go forward with the Chequers proposal because, frankly, there isn't an alternative that is put on the table by the critics of Chequers."
Labour MP Ian Murray, a supporter of the People's Vote campaign for a fresh EU referendum, said: "Westminster is back in session after a long summer recess, but it hasn't taken long for all hell to break loose again."