Mr Jeffrey Donaldson, the principal internal threat to Mr David Trimble's Ulster Unionist Party leadership, has threatened to quit if the party's ruling council fails to reject British and Irish government proposals to advance the peace process.
The Lagan Valley MP, who is at the vanguard of a large anti-Belfast Agreement rump in the UUP, said he would consider his position if the UUP's 900-strong ruling council failed to reject the declaration, which, he says, breaches key unionist principles.
The council meets on June 16th on a day that could see the party split down the middle with Mr Donaldson leaving. But Mr Trimble's leadership could become untenable if the declaration is rejected.
Mr Donaldson said the meeting represented a "defining moment" for the party.
Speaking on BBC Radio Ulster's Inside Politicshe said: "I believe the issue is so important that this is now a defining moment for unionism and the Ulster Unionist Party."
"I could not go before the electorate on a manifesto which either fudged or endorsed the joint declaration," he added.
The blueprint contains pledges on the scaling down of the British army presence in Northern Ireland, policing and justice reform, equality, human rights, a sanctions body for parties who contravene the Belfsat Agreement and a scheme that would allow so-called on-the-run paramilitaries to return home without facing jail.
Hardline unionists were outraged by the demilitarisation proposals and the scheme for paramilitaries on the run. Their anger was fuelled earlier this week by a leaked memorandum from the army's commanding officer in the North saying the Royal Irish Regiment home service battalions could be disbanded.
While the army has since pulled back insisting disbandment could only come if the paramilitary threat in Northern Ireland completely disappears, the leak prompted UUP hardliners to call a meeting of the party's ruling council.
Mr Donaldson said: "If my party decides that the Joint Declaration is a basis for moving forward, if they fail to unequivocally reject this Joint Declaration, then I think we may have reached the point where people like myself have to decide whether the Ulster Unionist Party is the best vehicle for persuing our objectives as unionists."
He added: "Even if they fudge the issue of the Joint Declaration, people like myself, the people who I represent - half the Ulster Unionist Party - we may be put in a position where we have to decide can we continue to give allegiance to a party that is either going to fudge or give a fair wind to a joint declaration that breaches key unionist principles and does not provide a basis for political stability."
PA