Tony Blair last night gave Northern Ireland's deadlocked political leaders only hours to agree a formula for salvaging the peace process.
He and his Irish counterpart, Bertie Ahern, told the parties – who still appeared at loggerheads – that time was running out to settle their differences.
Downing Street issued the warnings as the two prime ministers travelled to Weston Park, a stately home in the English Midlands, to host a fourth day of negotiations likely to stretch through the night.
Ahead of the resumed talks, Mr Blair's spokesman said: "The prime ministers want to establish whether they [the parties] have the will to move forward together." He echoed the Ulster Unionists and the nationalist SDLP by singling out decommissioning of paramilitary weapons as the crucial problem holding up progress.
The spokesman said the two leaders had full schedules after this weekend – Mr Ahern flies to South America on a 16-day trade mission on Monday while Mr Blair will travel to the G8 summit of world leaders on Friday. He said: "If you look at the two diaries there will not be another window of opportunity in weeks to come."
The tension surrounding the fresh round of talks was heightened by the violence in Northern Ireland after the 12 July celebrations, the climax of the Protestant marching season.
The Ulster Unionist leader, David Trimble, whose resignation as First Minister precipitated the current talks, is under pressure from Unionists to walk away from them unless he can wring concessions from the Republicans on decommissioning.
An added problem for the two governments is the refusal of the Progressive Unionist Party and the Ulster Democratic Party to attend the talks. Both have strong links to loyalist paramilitary groups.
The mood during the three days of talks earlier this week was almost unremittingly gloomy, but British officials insisted enough progress had been made to go on.
Sinn Fein – insisting that any deal has to be linked to more reform of policing in Northern Ireland and a scaling back of the Army's presence – also said it had had fruitful private discussions with the two governments on Wednesday.
However, the animosity between the parties surfaced again yesterday as Jeffrey Donaldson, one of the Unionist negotiating team, said the talks had been a waste of time, and the Sinn Fein president, Gerry Adams, said of the SDLP deputy leader, Seamus Mallon: "Seeking to score points off us in public while refusing to engage in private isn't the way to make progress."