Sinn Féin last night (Sunday) described as "baseless" suggestions that another senior party figure was a police informer at the height of the Troubles.
The DUP insisted that it would expose an alleged mole when politicians return from their summer break, provoking fears for the stability of the power-sharing executive.
The party alleges that the spy is a well known politician and conspired in the murder of a former police officer in Co Tyrone.
Upper Bann MP David Simpson, a cousin of the dead man, vowed to name the person in October under parliamentary privilege.
If he produces evidence to support the allegations the move could destabilise relations in the executive.
"At this moment in time we are keeping our powder dry," Mr Simpson said.
Frederick Lutton (40), a former part-time RUC reservist and National Trust caretaker, was shot dead in May 1979 as he left his car to lock the gates of the charity's Argory premises in Moy, Co Tyrone.
The last power-sharing executive dramatically fell after the arrest of Sinn Féin official Denis Donaldson (56) and others for alleged intelligence-gathering. Donaldson was later exposed as a police agent after charges were dropped. He was shot dead in Co Donegal.
Yesterday a Sinn Féin spokesman accused Mr Simpson of lacking "moral courage" by "hiding behind parliamentary privilege".
"You wouldn't judge the allegations. You would judge David Simpson and I would question his motivation in all this," he said.
