Mr David Trimble has given Mr Tony Blair a seven-day deadline to propose Sinn Féin's expulsion or face an Ulster Unionist withdrawal from the Northern Ireland Executive.
And while the British government grasped this apparent breathing space, some Whitehall sources accepted its only likely effect would be to force the British government to suspend the Executive and other institutions of government established under the Belfast Agreement either side of the weekend.
Speaking after Mr Trimble's one-hour meeting with the British prime minister at 10 Downing Street, the Northern Ireland Secretary, Dr John Reid, insisted London remained committed to maintaining the power-sharing administration, while admitting: "At the moment it's not easy to see how we find our way through."
However, ahead of tonight's potentially difficult talks between Mr Blair and the Taoiseach, Mr Ahern, Downing Street sources insisted the prime minister was not yet "resigned" to suspension.
Aware of Dublin doubts about last Friday's police raid on Sinn Féin's Stormont office and the scale and implications of alleged republican espionage at the heart of the Northern Ireland Office, Mr Blair's first priority will be to try and establish agreement on the nature of the crisis and its origins.
Mr Blair will also want to test Mr Ahern's apparent determination that fresh Assembly elections would be preferable to suspension.
And it seems clear that Downing Street will not go into decision-making mode until after Thursday's critical meeting between Mr Blair and Mr Gerry Adams, the Sinn Féin president.
However, authoritative unionist sources were insistent Mr Blair had "few options" other than suspension following Mr Trimble's demand that the British government table a motion proposing Sinn Féin's exclusion from office for debate by the Assembly by next Tuesday at the latest.
"In the event of that not happening," declared Mr Trimble, "that will leave us with no alternative but to remove ourselves from the administration."
Flanked by colleagues, the Ulster Unionist leader and First Minister warned: "I'm laying down the very clear parameters within which government has to act. Otherwise we will put an end-stop to this process."
Mr Trimble reaffirmed his willingness to share power with republicans, while insisting this could only now be on the basis of the Ulster Unionist Council's determination "that the issue of a private army had been resolved and the IRA disbanded". This seemed to encourage some small hope that Mr Blair's talks with Mr Adams might yet find a way to "get the \ transition and political tracks back in sync."
But asked if Mr Trimble had given Mr Blair scope for a negotiation with Mr Adams, the unionist sources told The Irish Times: "We've gone beyond that point. And it [any republican declaration of intent] would fall far short of what would be necessary in the present circumstances."
The same sources suggested Mr Trimble retained the option of tabling his own exclusion motion by Monday if Mr Blair had not reached the point of decision.
When they get to the point of discussing possible suspension tonight, Mr Blair is likely to argue it might present "the least damaging option" in terms of the shared commitment of the two governments to the principle of inclusivity which is the cornerstone of the Belfast Agreement - or, as one Whitehall source put it, "the least destructive of a series of destructive options". Suspension could be accompanied by a period of formal review which could take both governments and the parties into the new year and within sight of the scheduled date for fresh Assembly elections.
While waiting to hear what Mr Ahern has to say on the subject, it seems clear Mr Blair is unconvinced about the wisdom of rushing into fresh elections against the backdrop of political crisis.