Northern Ireland's First Minister Peter Robinson has requested the recall of the devolved Assembly after claiming a government deal that saw IRA terror suspects assured they would not be prosecuted also involved the granting of royal pardons.

Mr Robinson had already warned the Government he will resign unless a public inquiry is ordered into the controversy that has been triggered by the collapse of the Hyde Park bomb case.

Tonight the Democratic Unionist leader said the contents of a motion he would put before the Stormont Assembly during a specially convened session on Friday would depend on how the Government responded to his demand.

Mr Robinson issued the effective deadline ultimatum as he emerged from a meeting with Northern Ireland Secretary Theresa Villiers at Hillsborough Castle.

He earlier expressed outrage that he and other Stormont ministers had been left in the dark over the secret deal that saw government letters of assurance given to more than 180 Irish republican paramilitary suspects which led them to believe they would not be prosecuted.

Mr Robinson said he now understood pardons had also been granted.

"It appears that we are not just dealing with on-the-runs who received letters but we are also dealing with people who received the Royal Prerogative of Mercy, that indicates there were offences involved," he said.

"So we are not talking just about people who it is believed that the police did not have sufficient evidence to make a prosecution stick - that makes it a very serious matter."

The details of the deal emerged during the failed prosecution of John Downey, from Donegal, over the Hyde Park attack.

The 62-year-old denied murdering four soldiers in the 1982 bombing.

The case against him was ended because government officials mistakenly sent him a letter in 2007 telling him he was no longer a wanted man.

Prime Minister David Cameron condemned actions that led to the withdrawal of the prosecution case as a "dreadful mistake".

While the mistakes that resulted in Mr Downey receiving the letter have led to an apology from the Police Service of Northern Ireland (PSNI), a political crisis has erupted at Stormont over the wider issue of why the Government was sending out any assurance letters and allegedly without the knowledge of the majority of Stormont politicians.

The deal was apparently struck between Sinn Fein and the previous Labour government after an effort to legislate on the issue of on-the-runs (OTRs) failed.

But of 187 letters sent out, 38 were issued since the coalition Government took power in 2010.

With justice powers having being devolved back to Stormont just before the last general election, the anger of many Assembly members has been intensified by the fact the policy seemingly continued to be pursued by the Northern Ireland Office when law and order responsibilities rested with the power-sharing Executive.

Mr Robinson expressed hope that Prime Minister David Cameron would response positively to his call for an inquiry into the wider issue.

"I am asking for what is necessary and I don't think the Conservative Party is a party that will want to defend or support or hide those who have been responsible for terrorism, so I have high hopes that I am not asking for the impossible," he said outside Hillsborough Castle.

"I think if you consider the Prime Minister's demeanour in the House of Commons today he is clearly very angry about the issue and I hope that takes the form of being prepared to do something that it's clear that we can get all the information out and we know exactly what happened and who was responsible."

Stormont Justice Minister David Ford also met Ms Villiers in a separate meeting tonight.

He said he had received assurances that his devolved department had played no role in sending the letters.

"The Department of Justice has not been involved in sending out any amnesty letters and it will not be involved in doing so as long as I remain minister," he said.

Mr Ford, the leader of the cross-community Alliance Party, also backed Mr Robinson's request to recall the Assembly.

A letter had been sent out to Mr Downey by the NIO on advice from the PSNI that he was not wanted in connection with any terror offences. However, Mr Downey was being actively sought by the Metropolitan police.

The recent legal wrangle raised questions with the PSNI which, the court heard, knew about the UK arrest warrant for Mr Downey but did nothing to correct the error of 2007 in sending the letter.

The judge at the Old Bailey, Mr Justice Sweeney, threw the case out after Mr Downey's lawyer successfully argued at the 11th hour that a trial would represent an abuse of process given the written assurances he had received.

The Crown has announced that it would not appeal against the decision.

Mr Cameron told MPs in the House of Commons today: "We should be absolutely clear - the man should never have received the letter that he received.

"It was a dreadful mistake and a mistake that we now need to have a rapid factual review to make sure that this cannot happen again.

"But, whatever happens, we have to stick to the principle that we are a country and a government under the rule of law."

Attorney General Dominic Grieve stood by his decision to charge Mr Downey.

Earlier, Mr Robinson said he could not be First Minister in the five-party power-sharing administration unless he received answers.

"I am not prepared to be the First Minister of a government that has found itself having salient facts relevant to matters that are devolved hidden from them," he said.

"That is not acceptable to me.

"I want to have a full judicial inquiry into who knew what, when they knew it and exactly what they did know at the time.

"I also want to ensure that the letters that have been sent out are rescinded."

On July 20 1982, a car bomb left in South Carriage Drive killed four soldiers as they rode through Hyde Park in central London to the Changing of the Guard.

The explosion killed Roy Bright, Dennis Daly, Simon Tipper and Jeffrey Young, and injured other members of the Royal Household Cavalry. Seven horses were also killed as the soldiers travelled from their barracks to Buckingham Palace. Another horse, Sefton, survived terrible injuries and became a national hero.

The investigation into the bombing led police to Downey, through fingerprints on parking tickets and a description given by witnesses of two men carrying out reconnaissance in the area before the attack.

An arrest warrant was issued, but it was decided not to seek Downey's extradition from the Irish Republic in 1989, in part due to the lack of strong evidence against him, the court was told.

Then, in 2007, Downey received assurance that he was not at risk of prosecution as part of the scheme run by the Northern Ireland police.

Northern Ireland chief constable Matt Baggott has apologised to the families, saying: "I deeply regret these failings, which should not have happened."

He said checks were under way on information processed by the force about other similar cases.

Yesterday Ms Villiers said police in the province should reflect on "the serious error".

The Northern Ireland Secretary made no public comment in the immediate aftermath of her meetings with Mr Ford and Mr Robinson tonight.

Sinn Fein Assembly member Alex Maskey said the DUP and others had to face up to the fact that the region's troubled past had to be dealt with.

He rejected Mr Robinson's claim that the issue represented a crisis for the power-sharing institutions.

"If other people want to call it a crisis it is up to them, but they are missing the point," he said.

"The issue they are trying to address (the past) will still be on the table and needs to be dealt with."

A spokeswoman from the Northern Ireland Office said: "The Secretary of State had a constructive meeting tonight with the First Minister.

"The First Minister set out his concerns and they discussed possible ways forward and Ms Villiers undertook to get back to them."

She added: "Separately the Secretary of State had a useful meeting with David Ford."