Stormont Assembly members will convene for a special debate today on the controversy over the Government's handling of on-the-run republican terror suspects.

The devolved legislature was recalled for the additional sitting following a request by Stormont First Minister Peter Robinson at the height of this week's political crisis surrounding a scheme that saw letters sent to more than 180 individuals advising them they could return to Northern Ireland without fear of prosecution.

When Mr Robinson made the announcement on Wednesday, shortly after he had threatened to resign over the issue, there were fears the future of the power-sharing executive would be on the line during the plenary session.

But those concerns receded last night when the Democratic Unionist leader withdrew his ultimatum in response to an announcement by Prime Minister David Cameron that he was ordering a judge-led review of the matter.

Mr Robinson, whose resignation would have seen the institutions fold, and likely a snap Assembly election, claimed assurances he had received from the Government about the letters had now rendered them effectively "worthless".

Sinn Fein, the other main partner in the mandatory five-party coalition, had accused the DUP and other political rivals of "grandstanding" on the issue and claimed the threat of collapsing the Executive was a ruse to distract the public from the fact they all already knew about the process to deal with the on-the-runs (OTRs).

Mr Robinson said the deluge of calls his party had received from victims and other members of the public demonstrated there was nothing "synthetic" about the crisis.

With Sinn Fein likely to be pitted against the majority of other MLAs in the Assembly, the plenary session at Stormont will still undoubtedly be heated.

As well as commissioning the review, the Government said it would be making clear to all those who had received a letter in the past, as part of a deal Sinn Fein struck with the previous Labour administration, that if evidence now existed, or emerged in the future, which linked them to an offence, they could be questioned or prosecuted.

Mr Robinson claimed that move represented a fundamental change to how the scheme had operated before.

"I think that makes it clear that they have a fairly worthless piece of paper," he said.

Details of 187 letters sent to OTRs emerged when the case against a man charged with the 1982 IRA Hyde Park bombing collapsed.

John Downey, 62, from Donegal, denied murdering four soldiers in the attack in London.

The case against him was ended because government officials mistakenly sent him one of the assurance letters in 2007 telling him he was no longer a wanted man.

But the collapse shone the light on the wider policy of sending such letters to on-the-runs, with many politicians in Northern Ireland reacting with fury, claiming the scheme was operating without their knowledge.

The judge appointed by Mr Cameron will report by the end of May.

Last night Sinn Fein Assembly Member Alex Maskey described the review announced by the Prime Minister as "unnecessary".

"This announcement is a political fig leaf for the DUP to try and get them off the hook they jumped on to over the past few days," he said.

Northern Ireland Secretary Theresa Villiers told the BBC Radio 4Today programme it was a matter of "great regret" that Mr Robinson had not been briefed on the scheme.

She said her predecessor, Owen Paterson, had told Sinn Fein that no new cases would be considered by the UK Government, but left it to them to decide whether to raise new cases with the devolved authorities.

"I was briefed on the scheme when I became Secretary of State," Ms Villiers said. "It was explained to me that my predecessor looked at it when he was appointed.

"It was made very clear that it was not an amnesty, it did not confer immunity."

Mrs Villiers said that, out of 38 cases pending when the Coalition came to office, 12 had received a letter.

"In hindsight, yes, it is a matter of great regret to me that, in particular, neither the First Minister or the Justice Minister was briefed on this. Because, actually, what my predecessor, Owen Paterson, had decided to do was to recommend to Sinn Fein that, if new cases arose, it was not appropriate for the Government to deal with them, because policing and justice had by then been devolved.

"At that point we should have informed the devolved authorities, but we left it to Sinn Fein if they wished to raise new cases to pass them on to the devolved authorities."

Stormont's Justice Minister David Ford said he had been told by the Northern Ireland Office (NIO) that the cases of five on-the-runs were still in the system being considered.

Mr Ford, who characterised the OTR scheme as a "grubby side deal", said he had been assured that the responsibility for deciding whether a letter was sent to those individuals rested with the NIO and not his department - despite earlier claims by the Government that the scheme was now a devolved matter.

"My understanding from the NIO is that there are potentially five cases still in the system," he said.

The minister said he understood the cases emerged in late 2012.

"The senior (NIO) official I spoke to thought there were five cases still under examination and that they were their responsibility," Mr Ford added.