UUP leader Mr Trimble today faced a fresh challenge to his decision to return to Government with Sinn Féin following the start to IRA decommissioning.
Hardliners within the 850-strong Ulster Unionist Council gathered together the required number of signatures to call a special meeting to discuss political developments.
The president of the UUC said he had received the requisition for an extraordinary meeting signed by 60 constituency association members.
The meeting, which under the UUC constitution must take place within 21 days, has been called "to discuss the party's participation in government of Northern Ireland in the light of recent political developments, and to consider proposals we will advance with regard to the same," he said.
A call for a council meeting was defeated at the weekend when the Ulster Unionist Party 120-strong executive met and gave a clear endorsement to Mr Trimble's decision to return to government.
Past meetings of the council - involving grassroot members of the party - have given much narrower backing to Mr Trimble.
The Assembly is expected to meet at Stormont on Friday to elect a First and Deputy First Minister.
It appears it will be a close call with Mr Trimble needing the backing of a majority of both nationalist and unionist Assembly members.
The outcome could hang on the decisions of two dissident members of his party Assembly group, Ms Pauline Armitage and Mr Peter Weir, who have indicated they may not support the party line.
Both say they want more detail about what was decommissioned by the IRA, and what the timetable is for further disarming.
Mr Weir said today the council meeting was being called for because "people want to have their say about how things should go ahead."
Still sceptical about last week's move by the IRA, he added: "We need more clarity from General John de Chastelain (head of the international decommissioning body) and the IRA and from the Government about what price has been paid for decommissioning."
He said if Mr Trimble was re-elected as First Minister on Friday the council would consider whether the party should remain in government. If he was not, it would consider whether there should be a return to power-sharing.
Mr Weir predicted the most likely date for the council meeting would be November 17th, when the party conference is due to be held.
PA