Mr Jeffrey Donaldson used his address to an Orange demonstration to appeal for broad unionist opposition to the Joint Declaration and to challenge to the leadership tactics of Mr David Trimble.
The Lagan Valley MP, along with two others, has resigned the party whip at Westminster and last week was reinstated in the UUP after disciplinary moves against the three instigated by Mr Trimble were ruled unjust by the High Court.
Mr Donaldson told Orangemen that his dire predictions about the Belfast Agreement had been proven correct.
"I warned that permitting Irish republicans to take up ministerial office without an end to their terrorism would result in the corruptions of the democratic process. How right I was," he said.
"We now know that Sinn Féin/IRA abused their privileged positions at the heart of government to spy on the Northern Ireland Office and other political parties and to target politicians, police officers, prison officers and senior army personnel." Terrorism remains their stock-in-trade, he added.
Mr Donaldson said: "The result of this has been the collapse of the Assembly and the cancellation of the elections whilst the decommissioning process has descended into farce." Referring to the collapse of support for the agreement among Protestants, Mr Donaldson claimed 70 per cent of them would no longer vote for it.
"The answer to this collapse in unionist confidence from the \ government is to make further concessions to Sinn Féin/IRA in the form the Joint Declaration published in partnership with the Irish government." He attacked the declaration and its provision of a "significant role for the Irish government in the internal affairs of the Northern Ireland Assembly", which he said represented "a clear breach of a fundamental unionist principle".
Turning his fire on his party leader, he said: "Now for the plain speaking. David Trimble has refused to reject the Joint Declaration. In doing so he has split his party down the middle and is now embarked on a vendetta against those who oppose his high-risk policy."
He continued: "There is now a real danger that unless David Trimble revises his position and draws back from the brink, that the Ulster Unionist Party will implode and lose its position as the majority voice in unionism. If David Trimble insists on pursuing a course of action that will result in three of his MPs being thrown out of the party because of their principled stand, then the inevitable consequence of such action will be a formal and irreconcilable split in the party."
He claimed a wide support base at member level and, crucially, at higher ranks within the UUP. "There is a group of people in the centre of the party who share my concerns about the Joint Declaration but who to date have supported David Trimble's positions," he said. "I now make a direct appeal to those influential members to act now before it is too late."
Without being specific about his call to action, he added: "I cannot believe that these senior figures share David Trimble's view that the only way to resolve our problems is through the exclusion of the dissenting voices. They must know that this will result in the deepening of the crisis within the party and in our electoral demise."
He concluded: "The moment of truth has come and the challenge is there. Are we going to allow one man to take us down a road that the majority of unionists have no desire to travel? That way leads to political demise. It is time for Ulster Unionism to respond to the cry from ordinary unionists for greater unity and stronger politics. Only when we achieve this can we hope to address and resolve the wider political issues and reach agreement with others, including nationalists, on an end to paramilitarism and political instability. This is indeed a defining moment for unionism."