DUP leader Arlene Foster has said her party will "review" the confidence and supply agreement with the Conservative Party if Theresa May's Brexit deal passes parliament.
Mrs Foster, appearing on The Andrew Marr Show the day after the DUP conference in Belfast, said there were no circumstances under which her party would support the current deal.
"There is very much a border down the Irish Sea as a result of this and that's way we can't support this deal," she said.
The MLA said the DUP's confidence and supply agreement with the Conservatives remained "live" but again warned her party would "review" the arrangement if the Brexit deal gained parliamentary support, but she stopped short of saying she would definitely end the relationship with the Tories in those circumstances.
"We have to see where we are when that vote is taken," she said.
Asked if she would support the Prime Minister if a parliamentary defeat triggered a confidence vote in the Government, Mrs Foster added: "We will have to see what happens at that time. I think this last couple of weeks should tell all of us that you shouldn't jump ahead of ourselves.
"What we should do is wait to see what develops in that respect."
The DUP leader insisted it was not a choice between Ms May's deal or no deal.
"I believe we should use the time now to look for a third way, a different way, a better way."
She added: "We shouldn't just accept an outcome for the sake of it - we should try to get a deal that is good for everybody."
Mrs Foster said she did not think the deal will pass parliament and urged the PM to look for a better deal rather than "wasting time."
She said: "It is still live [Confidence and supply agreement] but you should remember also that the deal was signed to deliver on Brexit and to do that in a way that had a shared principals between the Democratic Unionist Party and the Conservative Party. But of course, the reality is there are many in the Conservative Party who don't like what is on the table today either.
"If it came to the situation that parliament did decide, and there is no evidence they are going to, but if they did decide to back this deal then obviously we would have to review the confidence and supply agreement."
On Sunday morning the EU leaders endorsed Theresa May's deal.
Asked if the Prime Minister had let her down, the DUP leader replied: "We are disappointed with the way this has progressed, I have to say.
"I really do understand that she is a unionist, that she wants to get the best deal but actually what this deal does is go against everything she has said around all of that."
Pressed on whether a defeat of the EU deal in Parliament would spell the end of Ms May's tenure as Prime Minister, Ms Foster said: "That's not a matter for me, that's a matter for her own party."