Mr Jeffrey Donaldson has warned the British government its attempts to rebuild the political process around the Joint Declaration this autumn are naïve.
In a speech delivered in Belfast last night, the Lagan Valley MP, facing disciplinary procedures for resigning his party's whip because of his opposition to the declaration, warned that wider consensus was needed.
He said majority support among nationalists and unionists was needed if political stability and devolution were to succeed.
"Over 70 per cent of unionists oppose the Joint Declaration," he said. "And the minority led by David Trimble will not be able to deliver unionist consent for any such deal".
He said that the same 70 per cent included those unionists who had voted for the Belfast Agreement in 1998 but who had now lost confidence in the process.
Mr Donaldson referred repeatedly to "one last chance to save the agreement" but warned that unionist trust, some form of DUP involvement and an end to all paramilitary activity were needed.
Unionism was currently dysfunctional, he said, and only a united unionist approach could act as an effective counterbalance to the political influence of Sinn Féin.
"It would be madness for unionists to return to the negotiating table until these problems are properly addressed and hopefully resolved. Then we will be in a much better position to reach agreement with the other parties and the government."
He repeated calls for the end to inquiries into alleged state misdemeanours such as the Bloody Sunday tribunal, and accused republicans of trying to take the past with them while claiming to move forward.