Latest News About England: Ministers 'urgently' considering inquiry into IRA fugitives row, Nick Clegg says

Deputy PM promises that government will 'do everything to get to the bottom of exactly what did and didn't happen'

Nick Clegg has said ministers are urgently considering a review into secret letters giving immunity to Northern Ireland terrorist suspects and suggested it would not be easy to stop them being issued.

The existence of the letters, effectively granting pardons to some IRA suspects still wanted for crimes during the Troubles – known as on-the-runs – has prompted a crisis in Northern Ireland, as unionist politicians appear to have been kept in the dark.

In an escalation of the row, Peter Robinson, the first minister, has threatened to resign unless there is an explanation from Westminster on Thursday and a judicial review into what happened.

Speaking on his LBC 97.3 radio show, the deputy prime minister said he had known about the letters for some months, even though Robinson only found out on Tuesday.

Former unionist leader Lord Trimble, an ex-first minister of Northern Ireland, who was instrumental in the peace process, also said he knew nothing about the deals, which became public during the collapse of the Hyde Park bombing trial when suspect John Downey produced a letter from police claiming immunity.

However, Clegg also suggested it might not easy to simply stop sending the letters, arguing "legacy" arrangements could not be casually disregarded.

"We'll urgently look at a [public inquiry] because I certainly appreciate how serious this is, how this provokes very strong emotions, dismay, outrage amongst the victims' relatives and families, so we do want to make sure that we do everything to get to the bottom of exactly what did and didn't happen," the deputy prime minister said.

He claimed the coalition had always been against letters being sent telling terrorist suspects they were not at risk of prosecution, even though reports suggest 38 have been issued since 2010.

"We all knew as a government, if you like, when we came into office in 2010," he told Sky News. "We knew this was a legacy issue which had been established by the previous government.

"My party and, indeed, the Conservative party, so both parties in this coalition, have been on record as being very against the proposal by the previous Labour government, for instance, to legislate for a wider amnesty and we want to see the rule of law always be applied. People must be held to account for their actions under the rule of law and in the case of Mr Downey clearly a dreadful mistake was made by him receiving that letter."

Speaking to BBC Radio 4's Today programme, Trimble said unionist politicians were not told the truth.

He said at some point during the peace process "an administrative process that operated independently of government" came into operation. "I would dearly love to know who signed off those letters," he added.

Robinson's resignation would trigger the collapse of the devolved five-party coalition dominated by his Democratic Unionist party and Sinn Féin.

The first minister has called for an emergency debate on the issue in the Northern Ireland assembly on Friday. He said he was consulting other parties about a motion in the Stormont parliament.

After talks with the Northern Ireland secretary, Theresa Villiers, at Hillsborough Castle on Wednesday night, Robinson also claimed that some of the 187 IRA on-the-runs wanted by police for past Troubles-related crimes had been given royal pardons, effectively granting them amnesties.

The DUP leader said: "We are not on the brink of a crisis – we are in a crisis." The scheme had created "a crisis of confidence that the people of Northern Ireland will have on the policing and judicial processes. And they are right to be angry."

The judge's decision to free Downey at the Old Bailey on Tuesday has resulted in the most dangerous political destabilisation at Stormont since devolution was restored in 2007.

North Belfast DUP MP Nigel Dodds said the amnesties, kept secret from unionists in negotiations leading up to the 2006 St Andrew's agreement, would have "very, very serious implications for Northern Ireland devolution".

Downey, from County Donegal, had been charged with the murders of four soldiers who died in the Hyde Park bomb in 1982, after his arrest at Gatwick airport last year. He strongly denied all the charges put to him and pointed out that he was a strong backer of the peace process.

The case against him collapsed after it emerged in court that he had a letter from 2007 that mistakenly suggested he was immune from prosecution over the Hyde Park atrocity.

Deputy first minister and Sinn Féin negotiator Martin McGuinness appealed for unionists to calm down over the controversy, saying: "No sensible person will thank anyone for threatening these institutions."

His party colleague Gerry Kelly, who bombed the Old Bailey in 1973, accused unionists of electioneering over their threats to pull out of the regional government. Kelly said the letters to the on-the-runs, which also include high-profile fugitives such as Sinn Féin's former press director Rita O'Hare, had been a pragmatic and necessary move to push the peace process forward.

Security sources in Northern Ireland told the Guardian the on-the-runs scheme was part of negotiations aimed at winning support within the IRA for the decommissioning of its arms – a key unionist demand during talks leading up to a political settlement.

But even strong supporters of the peace process, such as the former deputy first minister and Foyle MP for the nationalist SDLP, Mark Durkan, criticised the covert nature of the deal for on-the-runs.

In a pointed reference to former Northern Ireland secretary Peter Hain, Durkan told the House of Commons the deal had "proved that some of us were right when we warned the right honourable member for Neath [Hain] and others that they were blighting the peace process with their penchant for side deals, pseudo-deals, sub-deals, shabby deals and secret deals – which are now doing major damage to the process more widely."

Rowena Mason

Henry McDonald