Sir David Frost talks to David Trimble about the chance of success for the review

His comments come on the same day that the former US senator, George Mitchell, who chaired the Northern Ireland peace talks that led to the agreement, is due to arrive in the province to try to revive the deal.

The former senator is to meet representatives of all the parties in the assembly on Monday and Tuesday, at the start of a review of the implementation of the agreement.

This has come under strain amid mounting political tension in the province.

Speaking on BBC One's Breakfast With Frost programme, Mr Trimble called for the review to focus on whether paramilitaries have "any commitment to peace and democracy at all".

He said: "I'm not optimistic about the outcome of it because it seems to me we have a very serious problem, a more serious problem than we had in July."

He also said he was deeply concerned at reports on the Patten Commission's findings on policing in Northern Ireland, which are due to be published on Thursday.

David Trimble: Not optimistic about the outcome of review

He warned of the consequences of any attempt to include paramilitaries in any way in the new police service.

Meanwhile, the Social Democratic and Labour Party's Seamus Mallon has called for all the parties in the review to "go for broke" in trying to set up an executive.

The SDLP deputy leader accused all sides of breaking the Good Friday Agreement and said the review should not be used for recriminations.

He told Sky News' Sunday with Adam Boulton programme: "There's no point in recrimination, there is no point in each party trying to put the other in the dock.

"We were within touching distance of getting this executive set up. We must go for broke at it," he said.

'Fairly intense meeting'

On Saturday Sinn Fein followed the Ulster Unionist Party in agreeing to take part in the review.

Sinn Fein vice-president Pat Doherty: "There are many problems to be resolved"

The decision came in a vote after a two-hour "fairly intense" meeting of the Sinn Fein's national executive, or Ard Comhairle, in Dublin.

Consent was given but members expressed "deep concern".

But the chairman of Sinn Fein, Mitchel McLaughlin said they would be going into the review process to make it work despite Mr Trimble's reservations.

He said: "Well I think if David Trimble were to honour the commitments he made on behalf of his party then we would have had an executive 14 months ago.

"We could all have examined each other's credentials making the political institutions work that were designed in the Good Friday agreement.

"So I don't think David Trimble is in a position to set any tests for anybody until he passes a few himself."

'Terrible tragedy'

Meanwhile George Mitchell said it would be "unforgivable" if politicians allowed the Good Friday Agreement to fail.

The BBC's David Eades: " Mitchell returns to find the Good Friday Agreement hanging by a thread"

Speaking in an interview for the LWT Dimbleby programme on Sunday, Mr Mitchell said failure would be a "terrible tragedy and an irony".

He said he believed that a deal could be struck because the alternative was unacceptable.

"It is unthinkable to me that after having reached agreement, the parties who support the agreement will permit it to fail. That would be a terrible tragedy, an irony, and I think it would be unforgivable."

Tony Blair: "We still remain with the fundamental problem of trust"

UK Prime Minister Tony Blair also said there remained a fundamental problem of trust among the political parties.

In an interview for BBC Radio Scotland, he urged all sides to remember how far they had come and to consider the consequences of throwing it all away.

But Mr Blair also insisted the process was not unravelling.