Ulster Unionist leader Robin Swann has blasted the inclusion of the backstop in the withdrawal agreement as a “monumental error in judgment”.
He said his party “cannot and will not” support the withdrawal agreement.
The proposed deal has sparked chaos within the Conservative Party since it was unveiled on Wednesday.
Prime Minister Theresa May has suffered the loss of four ministers and faces continued speculation that a vote of no confidence in her leadership could be triggered by Tory MPs within days.
Mr Swann, whose party includes peers but no MPs, has claimed the withdrawal agreement will see Northern Ireland “potentially drift farther away” from the rest of the United Kingdom.
“In the unlikely event that the agreement makes its way through Parliament, its impact will be felt for decades to come,” he said.
“The impact may not necessarily happen immediately but as the months and years pass, the implementation of the disastrous backstop will see Northern Ireland potentially drift farther away from the rest of the United Kingdom unless action is taken to reverse its effects.
“It will act as an incentive for Scottish Nationalists who will use every excuse under the sun to destabilise another part of the United Kingdom.”
Mr Swann criticised the Democratic Unionists for the agreement which he said had “happened on their watch”.
“They failed in their primary duty to protect the integrity of the Union and its people,” he said.
“They told the people of Northern Ireland that they had the ear of the Prime Minister, they said that they had influence at the heart of the UK government.
“The DUP were asleep at the wheel. They bear enormous responsibility for what has happened.
“They either misused whatever influence they had or were so full of their own hubris, that they didn`t see what was coming down the tracks.”
Mr Swann also had stern words for the Irish Government, just hours before he was due to address an event at Fine Gael’s conference.
He is set to address an event titled Northern Ireland: Building Better Relations which will be attended by Irish Justice Minister Charlie Flanagan and Business Minister Heather Humphreys.
“The Irish Government should also be careful what it wishes for,” he said.
“Despite repeated warnings to tone down the language and act like good neighbours, the brash behaviour of the Irish Government led by Leo Varadkar and Simon Coveney may yet lead to a place where none of us want to go.
“A no-deal Brexit isn’t in anyone’s interests, but if they continue to pursue an aggressive stance in future negotiations, they will continue to raise the hackles of even the most mild mannered of unionists across the United Kingdom.
“For us, the Union between Great Britain and Northern Ireland comes first.”