Arlene Foster, leader of the Democratic Union Party, has asked British Prime Minister Boris Johnson to return to Brussels and renegotiate his agreement with European leaders to leave the EU.
Speaking at the Northern Ireland party's annual conference, Foster said the DUP and its 10 deputies could not support the Johnson agreement because it created a customs border in the Irish Sea.
"We will not support the government when we believe they are fundamentally wrong," she said.
She promised that the DUP, which two years ago agreed to support the conservative government for a parliamentary majority, would use its votes to ensure Brexit legislation failed.
Foster also said the DUP was ready for a general election, suggesting the party might support Johnson's call 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 the party would not support a deal that threatened the union of Britain and Northern Ireland.
Johnson will ask lawmakers on Monday to agree to hold an election on December 12, provided the EU agrees to grant Britain an extension beyond the Brexit deadline of October 31.
It is unclear whether the opposition Labor Party will support the motion, which requires two-thirds of parliamentarians to support it to pass.
Labor leader Jeremy Corbyn has said he wants to ensure that the EU has agreed to an extension before voting in an election to prevent the UK from falling without a deal on 31 October.