Prominent Tory Brexiter Jacob Rees-Mogg has said that he will vote against Theresa May's deal if it is brought back - unless the DUP are on board.
Earlier Mr Rees-Mogg had said he would back May's deal in a third meaningful vote, but qualified his decision on ITV's Peston.
He said it would be "very surprised" if the vote got brought back unless the government were confident it could be passed, which would mean addressing the DUP's concerns about the backstop.
The ERG chair said the one solution to get the Democratic Unionist Party to back Mrs May's deal would be for the UK to continue implementing EU law across the UK and Northern Ireland "temporarily".
He ruled out standing for leader, suggesting he might support Boris Johnson in the event of a leadership contest after Mrs May announced on Wednesday she would resign if her withdrawal agreement was passed.
Mr Rees-Mogg said he would not make any firm commitment to who he would back in the event of a leadership contest until next week.
However, he added that if Labour was to raise a no-confidence motion in the prime minister, he would fight it all the way, telling Peston: “I am Conservative to my fingertips, toenails, ends of my hair…”
Mr Rees-Mogg told Robert Peston: "I've always thought the union of the United Kingdom is more important than the European Union and maintaining the integrity of our country is of the utmost importance.
"So I'm strongly influenced by the DUP's concerns about the union in the backstop and I want to see their concerns addressed in a way that gives them thorough reassurance.
"I've said I will support the DUP if the deal is brought back and they are still opposing it."
In contrast, Chief Secretary to the Treasury Liz Truss said Parliament does not have any other option than to support Theresa May's deal.
Ms Truss told ITV's Peston programme she believes the Prime Minister will bring back her Withdrawal Agreement for a third meaningful vote later this week and thinks "it is the best way forward".
While her preferred choice remained the prime minister's deal, Ms Truss told Robert Peston she would prefer a no-deal to no Brexit.
"We have to get on with it," she said.