Theresa May heads to Brussels on Saturday with her Brexit deal under threat at home and abroad.
The Prime Minister will hold talks with European Commission president Jean-Claude Juncker before a summit of EU leaders on Sunday which is expected to endorse the deal thrashed out between negotiators from the two sides.
But Spanish Prime Minister Pedro Sanchez has threatened to “veto” progress without further guarantees for Madrid over the status of Gibraltar.
DUP leader Arlene Foster warned that the confidence and supply deal propping up Mrs May’s minority administration would have to be “revisited” if her Brexit deal gets through Parliament.
The DUP has strongly opposed the deal and the guest star at its conference will be Boris Johnson, a prominent critic of the Prime Minister’s approach and a potential rival for the Tory leadership.
Mr Sanchez has suggested that Sunday’s summit could be scrapped unless there is a breakthrough.
Mrs May has insisted her deal is in the interests of “the whole UK family” including Gibraltar.
The last-minute diplomatic spat comes as Mrs May’s critics laid bare the scale of their opposition to the Brexit plan.
Former Brexit secretary Dominic Raab, a committed Leaver who quit last week over the Withdrawal Agreement, said he believed the terms were so bad the UK would be better off remaining in the EU.
“I’m not going to advocate staying in the EU,” he told the BBC Radio 4 Today programme.
“But if you just presented me terms, this deal or EU membership, because we would effectively be bound by the same rules but without the control or voice over them, yes, I think this would be even worse than that.”
With more than 80 Conservative MPs – from both the Leave and Remain sides – threatening to vote against the agreement, Mr Raab warned it was unlikely to get through the Commons and said ministers should consider leaving without a deal.
But if it did pass the Commons, the repercussions could bring down Mrs May’s Government, with the DUP hinting at withdrawing the support of its MPs.
The DUP’s 10 MPs have proved reluctant to vote with the Government since the terms of the Brexit deal became known and the termination of their Westminster arrangement would be a major blow to the Prime Minister.
Mrs Foster told the BBC: “If she is successful in Parliament, and there is no evidence that she is going to be successful in Parliament, then of course we will have to revisit the confidence and supply agreement.”
But Mrs May, who launched an effort to sell the Brexit deal directly to the public, insisted that there would be no changes to appease critics at this late stage.
In a BBC phone-in she said: “If we were to go back to the European Union and say ‘People didn’t like that deal can we have another one?,’ I don’t think they are going to come to us and say ‘We will give you a better deal’.
“This is the deal that I think works for the UK.”
She warned rejection of her plan would lead to more “uncertainty and division” and that the public now wanted the Government and MPs to get on and deliver Brexit.
“In Parliament there’s a lot of focus on who’s going to vote for the deal or not, and outside I think people are thinking ‘Actually, let’s make sure we can get this through and get on with delivering’,” she said.
“If this deal doesn’t go through we are back at square one. What we end up with is more uncertainty and more division.”
“Personally, there is no question of no Brexit, because the Government needs to deliver on what people voted for in the referendum in 2016,” she said.
“As far as I am concerned, the UK is leaving the European Union on March 29 2019.”