Anti-agreement unionists are gathering signatures for a special meeting of the Ulster Unionist Council in preparation for a direct challenge to Mr David Trimble's leadership.
This emerged last night as the Northern Ireland Secretary, Dr Mo Mowlam, predicted the parties would meet the June 30th deadline for agreement on the decommissioning issue and the creation of the power-sharing executive.
One senior Ulster Unionist MP said a "no confidence" move against Mr Trimble could be triggered as early as next week, if the party's candidate, Mr Jim Nicholson, fares badly in Thursday's European election.
According to this source the determining issue in the timing of the anti-Trimble heave could be the percentage of the vote won by the Rev Ian Paisley and Mr Robert McCartney, and any evidence of a significant shift in unionist opinion against the agreement since last year's referendum and Assembly elections.
One of the prime movers for the special UUC meeting, a senior party officer, is known to favour challenging Mr Trimble should he agree to trigger the d'Hondt procedure for the designation of ministers without prior agreement on a process for IRA decommissioning.
However, another dissident MP said he believed Mr Nicholson would hold his seat, albeit with a reduced share of the vote, and that "the crunch" would be deferred until the basis for any agreement to create the executive became clear at the end of the month.
If these conflicting projections suggest some tactical disarray in the anti-Trimble camp, there is little doubt about the determination of his opponents to block any change in the party's declared position on decommissioning; or their belief that the fallout from last month's aborted Downing Street agreement has reduced Mr Trimble's room for manoeuvre.
One leading anti-agreement MP, Mr William Thompson, told The Irish Times: "He's not going to be able to sell anything but a substantial delivery of arms."
Opinion inside the anti-Trimble camp appears divided between those who believe the UUP leader would be prepared to enter the executive without clear commitments on IRA decommissioning and those who think he will eventually bow to "the reality" that he could not deliver his party for anything less. Mr Trimble seems to have been steadily losing ground since the 10-hour negotiation at Downing Street on May 14th, which resulted in an agreement rejected the next day by his Assembly party.
Some 48 hours later, on May 17th, five UUP MPs - Mr Thompson, Mr Clifford Forsythe, Mr Roy Beggs, the Rev Martin Smyth and Mr Jeffrey Donaldson - held a stormy meeting with the deputy leader, Mr John Taylor, at the House of Commons, at which the explicit threat was made that Mr Trimble would be removed as leader of the Parliamentary Party if he stuck to the terms of the Downing Street deal.
And at a meeting of party officers on May 22nd, before his much-criticised visit to the US and Canada, three Trimble "loyalists" joined forces with anti-agreement colleagues to oppose a move to trigger d'Hondt as a means of building pressure on Sinn Fein in the context of its consultations with the International Decommissioning Commission.
Mr Trimble has also been the subject of party criticism over his absence from Northern Ireland during the European contest. Party sources said his vulnerability had been illustrated by his inability to respond to demands from senior party officers for action to be taken against Mr Taylor, following his refusal to ask voters to give Mr Nicholson their first-preference votes.