Democratic Unionist Party leader Arlene Foster has urged UK Prime Minister Boris Johnson to go back to Brussels and re-negotiate his deal with European leaders to leave the EU.
Speaking at the Northern Irish party's annual conference, Foster said that the DUP and its 10 MPs could not support Johnson's deal as it created a customs border in the Irish sea.
"We will not give support to the government when we believe they are fundamentally wrong," she said.
She promised that the DUP, which has for two years agreed to support the Conservative government in order to give the party a parliamentary majority, would use its votes to ensure that Brexit legislation would fail.
Foster also said that the DUP was ready for a general election, suggesting that the party may back the call by Johnson on Monday for Britain to go to the polls on December 12.
Foster's speech followed that of DUP deputy leader Nigel Dodds, who described Johnson's deal as "the worst of all worlds".
He said that the party would not support a deal that threatened the union of Britain and Northern Ireland.
Johnson will ask MPs on Monday to agree to hold an election on December 12 providing the EU agree to give Britain an extension beyond the October 31 Brexit deadline.
It is not clear whether the opposition Labour party will support the bill, which requires two-thirds of MPs to support it in order to pass.
Labour leader Jeremy Corbyn has said he wants to ensure that the EU has agreed an extension before voting for an election, to avoid the UK crashing out of the EU without a deal on October 31.