Mr Mallon accused the Ulster Unionists of walking away from the deal, which sets out structures for the future political make-up of Ireland.

Sinn Fein leader Gerry Adams joined Mr Mallon in condemnation of the Ulster Unionists over the latest deadlock in the Northern Ireland peace process.

Mr Adams has told of how he warned Tony Blair that the anticipated advance the prime minister helped engineer this week would unravel.

Speaking ahead of talks in Dublin with Irish Prime Minister Bertie Ahern about the impasse over implementing the peace agreement, Mr Adams said he had been the last political party leader to speak to Mr Blair before he left Belfast after late-night negotiations, on Thursday.

"He said to me 'There will be an agreement by tonight', and I said 'Don't bet on it. In your absence the unionists will seek to unravel all of this'."

Gerry Adams: "The Unionists unravelled the deal"

Mr Adams added: "The unionists, when Mr Blair was here, went technically through the motions, and when he went away they unravelled them."

Mr Blair spent hours on Wednesday in discussions with politicians at Stormont in a bid to force progress towards the establishment of cross-border bodies and administrative departments outlined by the Good Friday Peace Agreement..

He left the talks forecasting an imminent breakthrough, and in Dublin Mr Ahern spent Thursday on standby, ready to travel to Northern Ireland to formalise any development.

But as the day drew on it became clear that a new deadlock had been reached, with the peace deal timetable looking set for new delays.

Mr Adams made it clear he blamed Northern Ireland First Minister David Trimble for the latest problems, claiming: "He is playing a slow, long hand, seeking to erode and dilute and hollow out the substance of this agreement."

David Trimble: Accused of "playing a long, slow hand" to erode the agreement

"Mr Blair has to focus and be very resilient. Coming in for five hours was not enough. There has to be a constant concerted focus on getting this sorted out."

The Sinn Fein leader said the unionists had not wanted Mr Ahern to travel to Belfast on Thursday: "Mr Trimble is not under pressure. He is very relaxed. He is now talking about this running into May."

"He is trying to protect the unionist ascendancy. If he can make peace in the process, he will do that - but on his terms."

Mr Ahern and Mr Adams were meeting on Friday in Dublin as efforts were renewed to overcome the latest setback in the Northern Ireland peace process.

'Unionists broke their word'

Seamus Mallon, deputy leader of the nationalist SDLP, also accused the Ulster Unionists of reneging on the deal.

"People did not honour their word. That's putting it as mildly as I possibly can," he told BBC Radio 4's The World At One.

Seamus Mallon: "There is very substantial anger"

"Apart from myself, there is very substantial anger among the other political parties at the way in which the Ulster Unionist Party seems to have walked away from an arrangement it had made and walked away within somewhere in the region of eight hours," he added.

"I was very concerned about it because I know what this delay is going to cause in terms of not being able to have the legislation ready in time to make the implementation bodies work.

"I know what it is going to do to the political process and I can see it even already today beginning to erode the credibility that the peace process in the assembly has had within the community," he said.

But Mr Mallon insisted that the agreement as negotiated would eventually be put in place.

'A load of rubbish'

Reg Empey: Firmly rejected criticism of the Ulster Unionists

Ulster Unionists hit back at critics who accused them of caving in to hard-liners in their midst by pulling back from a deal brokered by Mr Blair.

Senior party negotiator Reg Empey said: "I've never listened to such a load of rubbish as I have listened to over the past 24 hours in this matter - to think our party members are performing poodles who can be instructed to do this, that or the other by a certain time."

He also attacked the "spin doctors" of both the Irish and UK governments who believed they could "just whip everybody in" to suit their timetable.

"We've got to just realise that it is the politicians on the ground here who have got to do the business. Flying in or flying out is very helpful at times but it is not an end in itself.

"What people are trying to do is steamroller us and people are just not going to be treated like that."