Rightwingers in London and Belfast feigned outrage yesterday at a scheme to "let off" so-called on-the-run republicans that they had known about for five years.

British Prime Minister David Cameron pledged a judge-led inquiry into 187 letters assuring republicans that they would not be prosecuted if they returned to Northern Ireland.

He jumped on the bandwagon after Democratic Unionist Party (DUP) First Minister Peter Robinson threatened to quit, which would bring down the power-sharing agreement and trigger Northern Ireland Assembly elections.

But Sinn Fein Deputy First Minister Martin McGuinness said Mr Robinson was just trying to distract from the fact he and other unionists knew the deal existed.

The row over the letters erupted after the case against John Downey for the 1982 IRA Hyde Park bombing collapsed.

Government officials had mistakenly sent him a letter in 2007 telling him he was no longer a wanted man.

The DUP, the Ulster Unionist Party, the SDLP and the Alliance Party all claimed that they were unaware of the scheme and accused the government of striking secret deals with Sinn Fein.

Former Ulster Unionist first minister Lord Trimble insisted he knew "absolutely nothing" of the letter.

But claims by the DUP and other parties that they didn't know about the deal were shredded by the revelation that they were all briefed in detail on the scheme by a senior police officer in 2010, albeit without mention of the letters.

Members of all political parties, except Sinn Fein, were present at the Northern Ireland Policing Board meeting.

The issue was also specifically referred to in the high-profile Eames-Bradley report on dealing with the legacy of the Troubles, which was published in 2009.

Former Policing Board vice-chairman and report co-author Denis Bradley said he was surprised that unionists were claiming that they didn't know about the scheme.

Mr McGuinness said: "I think that the angst among unionist politicians is more centred around the common belief out there in society and in the media that they knew all about this.

"They may not have known about the letters, but they knew about the scheme and they knew that these people who were described as on-the-runs were being processed."

Mr McGuinness said letters were sent only to those people against whom police did not have evidence to prosecute.

He claimed the fact that other republicans had been unable to obtain letters proved there was no suggestion the documents amounted to an amnesty.

SDLP Foyle MP Mark Durkan said the case showed that his party was right to warn the previous Labour government that their penchant for "side deals, pseudo-deals, sub-deals, shabby deals and secret deals" would blight the Northern Ireland peace process.