The so-called ‘Star Chamber’ of lawyers convened by the pro-Brexit European Research Group (ERG) ound that agreements reached by the Prime Minister in 11th-hour talks in Strasbourg do not deliver the legally-binding changes the Commons has demanded.
A senior Conservative backbencher said rejection of the deal for a second time in a vote on Tuesday night could mean a snap election.
The judgment from Brexiteers came after Attorney General Geoffrey Cox told MPs that the changes "reduce the risk" that the UK could be trapped indefinitely in the backstop, but do not remove it altogether.
Responding to his advice, the ERG released conclusions which said: “Yesterday's documents considered individually and collectively do not deliver ‘legally binding changes’ to the WA (Withdrawal Agreement) or to the [backstop] Protocol.
“They fail to fulfil the commitment made by Government to the House in response to the Brady amendment ‘to obtain legally binding changes to the Withdrawal Agreement’.”
The changes to the deal “do not provide any exit mechanism from the Protocol which is under the UK's control”, the ERG added.
The DUP also said in a statement that its MPs would vote against the deal. Even if ERG MPs abstain, the government would still be expected to lose by dozens of votes.
Earlier, Cabinet gave its backing to the package of legal assurances secured in last-ditch talks in Strasbourg on Monday.
Mrs May told her ministers that passing the vote would allow the country to move on to a brighter future, while the alternative was uncertainty with no guarantee of what happens next.
She concluded the meeting by telling ministerial colleagues: “Today is the day. Let's get this done.”
But in a statement to MPs, her Attorney General said it was “a political judgment” whether the deal offered sufficient guarantees that the UK would be trapped under EU trade rules within the backstop.
He said “the legal risk remains unchanged” that if a trade deal with Brussels to supersede the backstop cannot be reached due to “intractable differences”, the UK would have “no internationally lawful means” of leaving the insurance policy for the Irish border without EU agreement.
In a statement to the Commons, Mr Cox said: “Were such a situation to occur, let me make it clear, the legal risk as I set it out in my letter of November 13 remains unchanged.”
Mr Cox told MPs: “There is no ultimate unilateral right out of this arrangement. The risk of that continues.
Charles Walker, the vice chairman of the 1922 Committee of backbench Tory MPs, said defeat in the meaningful vote would lead to a general election.
He told the BBC’s World At One programme: “If it doesn’t go through tonight, as sure as night follows day, there will be a general election within a matter of days or weeks.
“It is not sustainable, the current situation in Parliament.”