Rishi Sunak has been dealt a fresh blow as a group of hardline Tory Brexiteer MPs branded a key part of his deal with the EU "practically useless".
It comes as the Prime Minister prepares for a showdown in the Commons on Wednesday over his Windsor Framework agreement with the bloc.
The European Research Group (ERG) has not revealed how they will vote - but their highly critical report published on Tuesday is the strongest sign yet they will rebel.
Publishing advice from a so-called "star chamber" of lawyers to consider the deal, the ERG claimed EU law in Northern Ireland will "still be supreme".
They added the "Stormont brake" negotiated by Mr Sunak is "practically useless".
The mechanism is aimed at giving Northern Ireland politicians the power to stop new EU laws - but it can only be used in exceptional circumstances.
The ERG chief and Tory MP Mark Francois added: "The Framework itself has no exit, other than through a highly complex legal process."
Yesterday the Democratic Unionist Group (DUP) said its MPs would oppose the deal, adding the agreement does not deal with the "fundamental" issues.
But Mr Francois declined to say how the group would vote on Wednesday.
"The DUP have made their position very plain - they are going to vote against," he said.
"We, to some degree, have been critical of the Government for not allowing people enough time to digest everything," he said.
But noting legislation on the Stormont brake was only published on Monday, he said the group will not be "hoisted by our own petard".
"Because we need to allow people time to digest this, the ERG will be meeting again tomorrow," he added.
"We as a group will discuss what attitude, if any, to take and we will be having that meeting just before PMQs."
The Windsor Framework is designed to ease trade frictions and Unionists fears associated with the Northern Ireland Protocol, which was part of ex-PM's Boris Johnson's Brexit deal.
Sir James Duddridge, a Tory former minister and ally of Mr Johnson also told the Daily Telegraph he would "absolutely vote against" the deal that he is said to have labelled a "betrayal of Brexit".
Given Labour's support, the deal is all but certain to pass the Commons tomorrow - but the PM faces being humiliated if he is forced to rely on Labour MPs to get his deal through.
No10 on Tuesday argued that the Windsor Framework is the "only avenue" to change the current default, which is "automatic alignment".
The Prime Minister's official spokesman, asked if a vote against the Stormont brake is a vote for Northern Ireland to be aligned with EU rules, said: "Certainly on a factual basis, the legal default in domestic and international law is automatic alignment to EU standards and rules for goods, which is currently the case under the grace periods as well as the full implementation of the old protocol.
"So any change to EU rules on goods would automatically apply in Northern Ireland without any say for Stormont.
"So the framework is the only avenue by which that default can change.
"A vote against the brake, in factual terms, would lead to automatic alignment with the EU with no say at all."
