The North's First and Deputy First Ministers are at odds over the terms of the Belfast Agreement in the first serious rift between the Ulster Unionists and the SDLP since the pact was concluded on Good Friday.
The dispute concerns the establishment of the North-South Ministerial Council, charged with overseeing cross-Border co-operation. Mr Trimble had indicated that the body's inaugural meeting would be held within weeks: the deadline for agreeing the areas of cross-Border co-operation is October 31st.
But Mr Mallon made clear yesterday he would not assent to an inaugural meeting unless the North's "shadow" executive was set up first. Mr Trimble has delayed its formation because of his view that Sinn Fein cannot take part without prior IRA decommissioning.
Mr Mallon had refrained from public comment while his fellow Minister engaged in a war of words with Mr Gerry Adams on the issue. Comments from other senior SDLP figures since the weekend, however, showed all was not well between the main nationalist and unionist parties.
Finally yesterday, Mr Mallon broke his silence: "I have made it clear to the two governments, I have made it clear to Mr Trimble and everyone else that there must be a shadow executive, that I will not break the agreement by agreeing to a contrived situation to cover up the fact that the shadow executive has not been formed."
It there was no executive there could be no North-South inaugural meeting: "I will not agree to that and all decisions of a major type have got to be agreed jointly by the First and Deputy First Minister."
He added: "I am not challenging anyone. What I am simply saying is this is my role as Deputy First Minister." He would not break the terms of the agreement "to suit the party-political position of anyone in this process".
Mr Mallon said Mr Trimble and Mr Gerry Adams were both wrong on decommissioning. "Both have given an interpretation of this issue which is not within the agreement."
He agreed with the Taoiseach's call for a timetable for the disposal of arms and explosives. "That time-scale must tell us where it starts as well as where it finishes. I think that makes good common sense."
He said "battles of words" were being fought on the airwaves every day about the weapons issue and it had "almost become a soap opera".
Mr Trimble was flying to New York to attend a function but there was swift reaction from other leading Ulster Unionists. The UUP deputy leader, Mr John Taylor, claimed that by insisting a shadow executive be in place before the North-South Council met, Mr Mallon "is in non-compliance with the terms of the Belfast Agreement".
The UUP's Mr Reg Empey accused Mr Mallon of being "deliberately obstructive" and "threatening the potential for agreement on the North-South schemes before the October 31st deadline".
Mr Empey said it was "a supreme irony that we as unionists are pressing ahead with the NorthSouth Ministerial Council, we're holding discussions regularly about it and are willing to participate in it and we want to meet the deadline of October 31st, but people have manufactured a deadline that isn't there with regard to the formation of a shadow executive."
Welcoming Mr Mallon's remarks, Sinn Fein's Mr Martin McGuinness said: "Seamus Mallon is obviously dealing with the reality that we need to see as a matter of urgency the full implementation of the Good Friday Agreement."
Mr McGuinness, who was recently appointed as Sinn Fein's representative to the decommissioning body, said he had met the body's chairman, Gen John de Chastelain, on Wednesday. "We had a constructive and very realistic meeting," he said.