In the aftermath of the UK government's suspension of the Northern Ireland Assembly there is speculation that the Ulster Unionist Party exerted pressure on Northern Ireland Secretary Peter Mandelson to restore direct rule.

The BBC's Northern Ireland political editor Stephen Grimason says a post-dated resignation letter from UUP leader David Trimble was one of a number of factors which led to the decision to suspend devolution.

A senior Ulster Unionist ally of Mr Trimble's, Sir Reg Empey, told BBC Radio Ulster before his party's ruling council meeting on Saturday, that he was not aware of any behind-the-scenes threats.

But Stephen Grimason said he had no doubt the post-dated resignation letter David Trimble gave to the Ulster Unionist Party president, Sir Josias Cunningham, was being waved under Peter Mandelson's nose on Friday.

This was less than 24 hours before Mr Trimble had to face the 860-member UUP ruling council for a review of his decision to move into a power-sharing executive with Sinn Fein, in advance of any actual IRA decommissioning.

Sinn Fein reacted angrily to Mr Mandelson's decision to suspend the assembly and other political institutions.

The republicans said it was clear a second statement from the de Chastelain commission would have shown new movement on decommissioning.

Former Sinn Fein health minister Bairbre de Brun said Mr Mandelson allowed himself to be panicked by the Ulster Unionists into opting for suspension.

And although Stephen Grimason said Mr Trimble's resignation letter was not the only factor leading to suspension it was a factor. He told Radio Ulster: "From my understanding of the situation, the speaker's office was contacted at about 4pm.

"They were told that Josias Cunningham, the president of the UUP, would be there at 5pm to present David Trimble's resignation letter if there had not been agreement on suspension.

"This comes from the fact that the unionists thought they were possibly going to be double-crossed at some point during Friday evening.

"I think David Trimble didn't want to stand in front of his UUC meeting on Saturday with people having heard a half-baked version of events overnight and he would have to spend the first couple of hours of the meeting trying to talk them down from a ledge.

"I also am told that the secretary of state was told plainly that if this (suspension) was not on the Six O'Clock news, that would also trigger Mr Trimble's resignation."

Mr Grimason added: "Unionists were trying to back-stop themselves from being double-crossed, as they would see it, but at the same time I also understand the secretary of state did not believe he would get the clarity and plain language commitment from the IRA that he knew he needed to keep this all going.

"There is no doubt the unionists did put pressure on but it wasn't the only factor on the day."

Asked by the BBC if he felt he had in any way forced Peter Mandelson's hand by his presence at Stormont Josias Cunningham said at the UUC meeting: "I think that is for the Secretary of State to judge. I certainly felt was playing a small part in the process."