Twelve Conservative former ministers have written to Tory MPs urging them to vote against the Prime Minister's Brexit deal, according to reports.
The group, which is said to include Boris Johnson and former Brexit secretaries David Davis and Dominic Raab, said doing so may encourage the EU to come back to the negotiating table.
If Brussels refuses to reopen talks then MPs should be prepared to leave without a deal, they said in a letter.
Over 1,000 words long and reportedly sent to every Conservative MP, the letter also rejects Theresa May's warning that Brexit may not happen at all if her deal is chucked out.
The former ministers also deny that undermining the Prime Minister will lead to a general election and warned instead of "dire" consequences at the ballot box if her deal is passed.
According to the Daily Telegraph, the signatories include ex-party leader Iain Duncan Smith, Mrs May's former work and pensions secretary Esther McVey and Priti Patel, who was the PM's international development secretary.
The letter says that "nothing that has been said or achieved" since Mrs May shelved the original meaningful vote in December has changed the deal's main "weaknesses".
It cites the backstop and £39 billion "divorce bill" as major flaws of the Withdrawal Agreement, warning it is also a "blind alley" that risks handing power to Brussels.
"By voting down this flawed Withdrawal Agreement we can encourage the EU to renegotiate," the letter says.
It adds: "If it is the only way forward, we must have the confidence to be ready to leave on WTO (World Trade Organisation) terms.
"A managed WTO Brexit may give rise to some short-term inconvenience and disruption, but the much greater risks arise from being locked into a very bad deal."
The last-ditch attempt to secure and maximise Tory opposition to the PM's deal comes after four fellow Brexiteers said they will back the agreement, despite speculation they would vote against it.
The Tory MPs, including Sir Geoffrey Clifton-Brown and Sir Edward Leigh, said they would support the Government in the meaningful vote.
Former Public Accounts Committee chairman Sir Edward, who was appointed to the Privy Council last month, said it was "inconceivable" that Parliament and the Speaker John Bercow would allow the UK to crash out of the EU without a deal.
Tory former minister Andrew Murrison said he also had reservations, but had decided to back the deal.
And Conservative MP for Sleaford and North Hykeham, Caroline Johnson, said she was unhappy with some aspects of the deal and had intended to vote against it, but had changed her position.
Mrs May has also received the public support of a respected veteran Labour backbencher.
Sir Kevin Barron, a former Commons standards watchdog chief who has been MP for Rother Valley since 1983, said he was "truly saddened" that some MPs were trying to "overturn" the 2016 referendum result.
He told The Times: "I know it will disappoint many within my party but I will be voting in support of the prime minister's deal on Tuesday as that is the only option on the table that truly enacts the promises that I made to my constituents and avoids the horror of a no-deal Brexit."