Sinn Fein has accused Mr David Trimble of making a "massive blunder" in rejecting the International Independent Commission on Decomissioning's judgment that the IRA had begun a process of decommissioning.
Mr Martin McGuinness said the Ulster Unionist leader had made an error of judgment and should "butt out" of the commission's decisions, leaving it to get on with discussions with armed groups.
"Of all the mistakes made throughout the course of the last three years in the Good Friday agreement process, what David Trimble has done in rejecting the determination of the IICD is the greatest mistake of all," Mr McGuinness said.
He accused Mr Trimble of attempting to undermine the commission in his refusal to believe a process had started without actual decommissioning taking place.
Mr McGuinness said the commission was established under the Belfast Agreement to oversee decommissioning "and the politicians who agreed to the establishment of the IICD need to support that body and that commission".
"If people support the agreement, they have to support the decommissioning body. We in Sinn Fein do," he said.
"It's now time for David Trimble to butt out of the international body, to recognise that Gen de Chastelain and his colleagues should be left to do that work in conjunction with the armed groups.
"If he thinks he hastens the day that his version of the Good Friday agreement will be implemented by this strategy, then he is living in cloud-cuckoo-land."
Mr McGuinness called on the British government "to stop pandering to David Trimble and to begin defending the Good Friday agreement".
The IRA initiative had been the "greatest breakthrough" in the peace process, he said, and the question was how to build on that.
Mr McGuinness warned against any suspension of the institutions.
"There is no provision in the Good Friday agreement for suspension, so to suspend the Assembly is effectively to breach the Good Friday agreement."
He also called on Mr Trimble to rethink his strategy.
"If he were to join the club, if he were to join in with the rest of us, we could be much more hopeful and optimistic that we could get all of the armed groups in society to recognise they should make their own contribution," he said.