Arlene Foster has warned the Prime Minister not to roll over in the face of EU intransigence.
The DUP leader urged Theresa May to stand up to fellow European leaders and demand changes to the 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.
The party believes the measure would undermine the constitutional integrity of the UK by creating 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.”