Hardline Tory Brexiteers and DUP have joined forces to alert them to being ready to vote on EU plans to withdraw from Theresa May.
The move comes as the prime minister struggles to keep Brexit's agenda on track as he faces growing tensions with Tory and opposition reports from Brussels to a key place withdrawal proposals.
With the shock resignation of pro-Europe Transport Minister Jo Johnson continuing to intervene in the ranks of Tory, Mrs May does not have time to seal an exit agreement from the EU.
Tory Brexiteer, Steve Baker, deputy chairman of the 80-year European research team (ERG) of conservative backbenchers, and DUP spokesman Brewit, Sammy Wilson, said they would oppose any deal they considered threatening the union and could put a commercial border under the Irish Sea.
Writing at Sunday Telegraph, they said: "We share the Prime Minister's ambition for an EU Free Trade Agreement, but not at any price and certainly not at the cost of our union.
"If the government makes the historic mistake to give priority to the persistence in the EU for the establishment of an independent and the whole of the UK, then unfortunately we have to vote against the agreement."
The hope of signing the Cabinet for signing Brexit's proposals this week seemed to recede quickly as it was reported that the EU had rejected London's plans for an independent arbitration clause that could allow the United Kingdom to resign a backstop agreement at the border of Northern Ireland.
With the Tories both for and against the retreat becoming more vocal in their opposition to Mrs May's stance, Brexiteer arch Jacob Rees-Mogg called on the prime minister to change the party.
She called on May to end the deadlock by paying 20 billion euros to secure a "no deal" agreement with the block after the withdrawal.
Writing in Mail on Sunday, Mr Rees-Mogg, who is head of the ERG, proposed to offer the financial agreement in Brussels to "make the withdrawal as friendly as possible".
A former critic of the £ 39 billion divorce bill, the United Kingdom is going to pay to the EU, Rees-Mogg wrote: "It's time to convince convinced Brexiteers like me for a compromise.
"So at this late hour in the negotiations, we would like to make a new, generous bid to break the deadlock to achieve a No Deal Plus.
"It will cost us money, but will eventually break the Project Fear nightmare scenarios."
A government source told the press: "The end of the negotiations would always be tough.
"There are some issues that need to be resolved in Northern Ireland's attitude and these are the most difficult.
"They include ensuring that, if ever, it is not permanent and there is a mechanism to ensure that the United Kingdom can not be held indefinitely."
Former Secretary of State Justine Greening, who shares Mr Johnson's view that a new referendum is required, called on Tories to oppose the Prime Minister's proposals.
He told The Observer: "The parliamentary impasse is clear for some time, it is now crucial for Parliament to vote on this plan because it is the greatest gift of sovereignty in modern times."
After Mr. Johnson's resignation there were says other ministers are thinking of stepping down on Brexit.
The interrogators insisted that the United Kingdom should not be involved in a potentially permanent customs union agreement with the EU as a price for avoiding tough borders in Northern Ireland.