Mr Jeffrey Donaldson warned he may quit the Ulster Unionist Party if he loses a showdown over British and Irish plans for the Northern Ireland peace process.
It could be a move which would finally split the party down the middle.
The UUP's 900-strong ruling council meets on June 16th at the request of members opposed to the Belfast Agreement who want the council to reject the joint declaration plans published by the governments last month.
The Lagan Valley MP said if the council fails to reject the joint declaration he would consider his future in the party.
He said the meeting represented a "defining moment" for the Ulster Unionist Party.
The declaration, he insisted, breached key unionist principles and did not provide the basis for stability in the province.
Speaking on BBC Radio Ulster's Inside Politics programme, he said if the declaration was not rejected then "people like myself will have to consider our position.
"I could not go before the electorate on a manifesto which either fudged or endorsed the join declaration."
The blueprint contains pledges on the scaling down of the army presence in Northern Ireland, policing and justice reform, equality, human rights, a sanctions body for parties who contravene the Belfast Agreement and a scheme to allow so-called on-the-run paramilitaries to return home without facing jail.
Some unionists were further angered earlier this week by a leaked memorandum from the army saying that 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, it was too much for the hardliners in Mr Trimble's party.
They gathered together the required number of signatures to call a party ruling council meeting demanding that it reject the scheme.
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 pursuing our objectives as unionists."