DUP leader Arlene Foster last night warned the Prime Minister not to roll over in the face of EU intransigence.
Mrs Foster called on Theresa May to stand up to fellow European leaders and demand changes to the UK/EU Brexit Withdrawal Agreement.
Mrs Foster, whose party is keeping the Government in power through a confidence and supply deal at Westminster, has vowed to vote against the draft treaty if the contentious Irish border backstop is not binned.
Mrs Foster's uncompromising message to the Prime Minister comes after Mrs May spent two days in Brussels unsuccessfully pleading with EU leaders for "further assurances" which might help her persuade UK MPs to back her Brexit Withdrawal Agreement.
The DUP is vehemently opposed to the Withdrawal Agreement in its current form, which would see Northern Ireland kept inside the EU's regulatory framework if a wider trade deal between the UK and EU could not be agreed. The party believes the measure would create an economic border between Northern Ireland and Great Britain.
Responding to the EU's refusal to countenance changes to the withdrawal deal and Mrs May's failure to secure legal guarantees that the backstop would only ever be temporary, Mrs Foster said: "The Prime Minister has promised to get legally binding changes. The reaction by the EU is unsurprising. They are doing what they always do.
"The key question is whether the Prime Minister will stand up to them or whether she will roll over, as has happened previously.
"This is a difficulty of the Prime Minister's own making. A deal was signed off which the Prime Minister should have known would not gain the support of Parliament.
"If the Prime Minister had listened to our warnings and stood by her public commitments, we would not be in this situation."
Mrs May's hopes of a legally binding commitment on the backstop from the EU27 were dashed when Mr Juncker said there could be "no real changes" to the Withdrawal Agreement.
After listening to a presentation from the Prime Minister about MP's concerns, the EU27 leaders tore up a draft communique prepared by officials which would have offered Mrs May "further assurances".
But she said yesterday talks with EU officials about changes to the Withdrawal Agreement would continue, despite the rejection of her requests by EU leaders including Angela Merkel and Emmanuel Macron.
In a press conference in Brussels yesterday, the Prime Minister acknowledged that it would not be possible to reopen the agreement to alter the backstop provisions which have sparked mass rebellion among Tory MPs.
She confirmed she had a "robust" discussion with European Commission president Jean-Claude Juncker and insisted she had been "crystal clear" about the UK's need for firmer assurances that the backstop cannot become permanent.
"The EU is clear, as I am, that if we are going to leave with a deal, this is it," she said.
"But my discussions with colleagues today have shown that further clarification and discussion following the council's conclusions is in fact possible.
"There is work still to do and we will be holding talks in coming days about how to obtain the further assurances that the UK Parliament needs in order to be able to approve the deal," Mrs May said.
But Mr Juncker criticised Mrs May's lack of clarity over what she was seeking from the future UK/EU relationship.
He said: "We don't want the UK to think there can be any form of renegotiation, that is crystal clear.
"We can add clarifications but no real changes. There will be no legally binding obligations imposed on the withdrawal treaty."
Opposition leader Jeremy Corbyn said Mrs May's plan was "dead in the water" and reiterated Labour's call for a vote on it before Christmas.
Last night DUP Brexit spokesman Sammy Wilson MP hammered home his party's message to the Prime Minister.
Speaking at a rally for Brexit campaign group Leave Means Leave, the MP said that the assurances Mrs May was seeking from the EU were worthless.
"From the point of view of Northern Ireland, we're not going to accept a situation where we have to regard the rest of the UK as a third country: whether we find ourselves cut off from out main market; where we can't take part in trade deals; where we find that forever we will have our laws made in Brussels without any input or any ability to change them, regardless of how damaging they are.
"That's not democracy, that's dictatorship, and we'll not have it."
Northern Ireland-born Labour MP Kate Hoey also gave a speech where she said: "How could any UK Prime Minister agree a deal that separates out Northern Ireland from the rest of the UK? As someone who grew up in Northern Ireland, proud, very proud to be pro-Union, I feel very strongly about this.
"We didn't spend in Northern Ireland 30 years stopping IRA terrorists killing soldiers, police and civilians in order to get a united Ireland, to allow a few jumped-up EU bureaucrats and a complicit Prime Minister to try to do the same thing by the back door."