Speaking after a telephone conservation with the UK prime minister, he said: "A process like this was difficult to put together, and it is going to be difficult to implement.

"In this case I think the day was a bad one."

Mr Ahern indicated that the February deadline for having cross-Irish border bodies in place could now be at risk.

Northern Ireland Correspondent Denis Murray reports on the log-jam

Agreement on cross-border bodies was put on hold on Thursday night after a bitter row between unionists and nationalists following talks at Stormont.

British and Irish ministers were hoping a deal could be struck ahead of next week's Nobel peace prize presentation to Ulster Unionist Party leader and Northern Ireland First Minister David Trimble and John Hume, leader of the nationalist SDLP.

Sources close to Mr Blair, who flew to Ulster on Wednesday night to help break the deadlock, said: "He is disappointed that the parties had not been able to build on the progress made last night and that a number of different interpretations had arisen."

Northern Ireland Deputy First Minister Seamus Mallon - angry at opposition to progress - called on all parties to stand by proposals reached at Stormont.

Seamus Mallon: Angry

"As far as I am concerned my negotiations with the prime minister and the other parties have been completed on this basis.

"That is the deal that has been done and I stand by it. It is now for the parties to confirm their participation in the agreement brokered by the PM."

It had been thought the sides had virtually agreed arrangements for the creation of 10 departments at Stormont after a marathon session of negotiations involving Mr Blair.

Several cross-border institutions dealing with a range of issues from transport to inland waterways had also been agreed.

Under the plans, Sinn Fein would be entitled to two ministers in the new executive.

In return the SDLP and Sinn Fein are understood to have agreed to drop their demand for inward investment to be dealt with on a cross-border basis.

John Taylor: Accused of caving in to hardliners

But Ulster Unionist deputy leader John Taylor said his party was not yet ready to finalise the deal thrashed out with Mr Blair, prompting accusations of an internal dispute within the party.

Rivals accused the leadership of caving in to hardliners after a meeting of the UUP assembly party earlier.

Gerry Adams: "Excuses are being made for the delay"

Sinn Fein leader Gerry Adams said: "There has been some measure of progress reached and now some people are in denial and they are seeking to move backwards."

The sticking point for unionists is understood to be the proposed cross-border body on trade and business development, which is not mentioned in the Good Friday Agreement.

John Taylor: "Considerable work to be done"

Mr Taylor said negotiations would continue, possibly for days.

But he said there was no reason why the arrangements could not be finalised without Ulster Unionist leader David Trimble. He leaves Belfast for the United States on Friday and will then travel on to Oslo to receive his Nobel Peace Prize with SDLP leader John Hume next week.

Ian Paisley Junior: Agreement is a sham

"I don't expect any conclusion tonight (Thursday) or tomorrow or possibly well into next week."

The hardline Democratic Unionists, meanwhile, blamed the difficulties on deficiencies in the Good Friday Agreement.

Party spokesman Ian Paisley Junior said: "They are starting to run into the problems that have always been there and that is that this agreement is a sham."