Gavin Williamson has described Theresa May’s Brexit talks with Labour as a “grave mistake”.
The former defence secretary, who was sacked over the Huawei leak, said pressing ahead with the talks will have “fatal” consequences.
Writing in the Mail on Sunday, he labelled Mrs May as “politically naive” for going into fruitless negotiations which he claimed were bound to fail.
The Tory MP for South Staffordshire said doing a deal with Labour on Brexit “sounds so simple and so reasonable, but it is destined to fail”.
“Even if Labour do a deal, break bread with the Prime Minister and announce that both parties have reached an agreement, it can only ever end in tears,” he told the Sunday paper.
“The Labour Party does not exist to help the Conservative Party.
“Jeremy Corbyn will do all he can to divide, disrupt and frustrate the Conservatives in the hope of bringing down the Government.
“His goal, and he has made no secret of it, is to bring about a general election.”
Mr Williamson said the Prime Minister seemed oblivious to the fact many Tories believe she is “negotiating with the enemy”.
“There is a clue in their title: Her Majesty’s Official Opposition,” he said. “Their priority is to derail the Government.
“Even if we get to a point where Jeremy Corbyn agrees a deal with the Prime Minister, when it comes to detailed scrutiny of the votes, Labour will revert to form.
“Even if it passes the first few votes, it will fail later.”
The former Cabinet minister said there was a simple calculation that a deal could pass with the combined votes of Labour and Conservative MPs, but “tough realities” must be faced if the deal was “far removed” from expectations.
Mr Williamson said this could mean Mrs May has support from “less than half the Conservative MPs” including those “on the payroll”.
That would lead to “knife-edge votes” and a “number of defeats” due to “up to 80” Labour rebels who want another referendum, he said, alongside SNP, Lib Dem and Change UK MPs who have pledged to vote against.
He said: “This is when Labour will finally kill it, if they have not done so already.
“Labour will be able to credibly say it is not what was originally agreed between them and the Prime Minister.
“It is politically naive to go down this route.”
Warning Mrs May she was turning her own supporters against her, Mr Williamson said that scenario “should be avoided at all costs”.
He said: “The Prime Minister needs to recognise that futile efforts to pull off this Labour deal are damaging us all.
“It is a grave mistake for any Prime Minister to fail to recognise when a plan will not work and it is fatal to press on regardless.
“We need to accept that these talks with Labour are fruitless and that not only will they not deliver the Brexit that people voted for, they are a betrayal of the direct instructions the people gave us in 2016 and 2017.”
Urging Mrs May to “make the right choice”, the MP suggested new leadership was needed.
“We are now at a crossroads and it is imperative the Prime Minister makes the right choice,” he said.
“In order to deliver Brexit, there has to be a clear-sighted determination of what you are wanting to deliver, as opposed to delivering the lowest common denominator.
“The only way to deliver anything is by ensuring you have your own tribe and your own people with you 100% of the way.
“This is what has to be delivered – not doing a deal with Labour.”