DUP leader Arlene Foster today told Boris Johnson to renegotiate his Brexit deal if he wants her party's support in Parliament.
Ms Foster told her party conference in Belfast that the Prime Minister's Withdrawal Agreement Bill would take Northern Ireland in the "wrong direction".
She reflected on Mr Johnson's speech to the same conference last year, when he insisted he would not erect any new economic barriers in the Irish Sea.
And Ms Foster said the DUP has now put the Prime Minister on the "naughty step" after voting against his government twice in key votes this week.
She said: "With our 10 votes being so critical in Parliament, there is a great responsibility to make sure we support the right decisions for Northern Ireland - but have the boldness and strength to stand up and say no when we need to."
And Ms Foster made it clear that unless Mr Johnson's agreement with EU leaders changes, it will not have her party's support.
"Let me say clearly from this platform today," she said, "that we want to support a deal that works for the whole of the United Kingdom and which does not leave Northern Ireland behind.
"But without change, we will not vote for the Prime Minister's agreement.
"It would be bad for Northern Ireland economically and will weaken the foundations of this great United Kingdom."
