Senior unionists believe Mr David Trimble is set to return to the Mitchell review to give the green light to the deal allowing the formation of Northern Ireland's power-sharing executive at the end of this month.
However, there is also growing speculation within the party that the Ulster Unionist leader will resign if the IRA has not started a process of actual decommissioning by the end of January.
One Assembly member last night told The Irish Times he believed Mr Trimble could only carry the party with him on a "cast-iron promise" that some actual "product" would be delivered within eight weeks of the UUP entering government with Sinn Fein.
Another close Trimble ally said: "In the event of non-delivery I think David would recognise the realities of his position".
Mr Trimble is expected to talk to Mr Blair this morning before rejoining Senator George Mitchell and the other parties for the final stage of the review process.
If, as expected, he signals his readiness to run with the deal concluded last week with Sinn Fein, sources say the "sequencing" process - a series of statements by Mr Trimble and Mr Gerry Adams, the two governments, Senator Mitchell, Gen. John de Chastelain and the Provisional IRA - might be played out over the ensuing three or four days.
To that end it is believed preparations have been put in hand for a possible meeting of the policy-making Ulster Unionist executive for Thursday night or Friday afternoon.
A meeting of the party executive would be of huge strategic importance in Mr Trimble's battle to secure his party's support. No final decision has yet been taken about the timing of any meeting of the ruling 800-strong Ulster Unionist Council.
One source last night even suggested a council meeting might not in fact be necessary if Mr Trimble is able to sway doubtful Assembly members and win executive approval for the final published package. However, this seems unlikely, since anti-agreement unionists would have no difficulty obtaining the 60 signatures necessary to force an emergency council meeting.
Moreover, even some of Mr Trimble's supporters will oppose taking the issue to the council unless satisfied that the result will not be a seismic split in the party. And they say the key to securing the necessary degree of party unity will be an assurance from Mr Trimble about what would happen should the IRA fail to deliver on decommissioning.
The pressure on Mr Trimble to foreclose at that point is rooted in continuing, strong opposition to the existing plan contained in the package concluded last week that, in that event, the executive would be suspended and the process go into review.
One of Mr Trimble's most determined opponents said last night: "This is exactly what he rejected last July. And we all know that, having conceded the principle and let Sinn Fein into government without any guns being handed over, the whole point of the review would be to get the executive back on the road as quickly as possible."
In the proposed sequencing arrangement it is thought IRA confirmation that it is appointing an interlocutor to interface with the International Decommissioning Commission would swiftly follow Mr Trimble's pledge to trigger the d'Hondt process for the appointment of ministers in the week beginning November 29th.
Gen. de Chastelain would report by January 5th that agreement had been reached on the "modalities" for a decommissioning process, and again at the end of that month. It is understood that if he is unable by the end of January to report an actual start to decommissioning, the executive would fold and the review process would be restarted.
Senior British government sources at the weekend indicated their clear belief that the appointment of an IRA interlocutor and the commitment to agree modalities coupled with a Sinn Fein assertion that "there cannot be permanent peace without decommissioning" would constitute a clear commitment that actual decommissioning would occur.