The DUP has said peace in Northern Ireland is at risk if a Brexit deal draws a customs border in the Irish Sea.
DUP leader Arlene Foster is currently giving a speech at the party's annual conference in Belfast.
Speaking today, Ms Foster said that "on Brexit, we will not give support to the (British) government when we believe they are fundamentally wrong and acting in a way that is detrimental to Northern Ireland and taking us in the wrong direction.
"We will oppose them and we will use our votes to defeat them.
“Let me say clearly from this platform today 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.”
DUP MP Jeffrey Donaldson said the DUP wants Brexit to happen but not if the North's links with Britain are weakened.
I said a week ago, that this was not over and it isn't. We've stopped this deal in its tracks, that's clear.
"Our votes have been crucial in the House of Commons and we'll continue to use our ten votes to try and get the changes that we need to this deal.
"We want to go forward. We want Brexit to happen."
Meanwhile DUP deputy leader Nigel Dodds has told Boris Johnson his party will only support a Brexit deal that protects the Union.
Addressing his party conference in Belfast, Mr Dodds referenced Mr Johnson’s speech at the same event last year, when he pledged not to erect economic barriers in the Irish Sea.
“Stick to your word, Prime Minister,” Mr Dodds told delegates. “This party has always made clear that we want a deal.
“A deal that works for the whole of the United Kingdom and for the European Union. A deal that recognises Northern Ireland’s unique history and geography.
“A deal that supports our business community and families across this Province.
“But, this party will only support a deal that ensures Northern Ireland leaves the European Union in a way that protects both the economic and constitutional integrity of our United Kingdom.”
Mr Dodds said the current deal provided the “worst of both worlds”.