The political crisis in Northern Ireland deepened tonight after First Minister and DUP leader Peter Robinson met Secretary of State Teresa Villiers at Hillsborough. 

Mr Robinson said afterwards that he had demanded an independent public inquiry into how more than 200 letters were issued by the Northern Ireland Office, telling the recipients they were not wanted for questioning by the authorities.

The First Minister said he has given the British government until Friday to provide answers to a series of questions tabled during tonight's meeting. 

He is to seek the recall of Northern Ireland's Assembly on Friday and says the response of the British government will influence his threat to resign as First Minister of the executive.

According to Mr Robinson, he learned during his meeting with the Secretary of State tonight that a Royal Prerogative of Mercy had been provided to a number of republicans who were wanted in relation to a series of terrorist crimes during the Troubles.

He said public confidence in the judicial and political process has been undermined and full transparency in required in relation to the so-called on-the-runs controversy.

The developments follow the collapse of a trial in London yesterday.

Donegal man John Downey, who was charged in connection with 1982 Hyde Park bombing that left four British soldiers dead, walked free from the Old Bailey. 

Mr Robinson this afternoon said he was not prepared to remain as First Minister in a power-sharing administration that is being kept in the dark about such an important matter. 

Unionist politicians have requested the British government clarify if a deal was made with "on-the-run" republicans in 2007.

Under European Convention on Human Rights legislation, an individual is entitled to be told by the authorities if they are being sought in relation to criminal proceedings.

The PSNI was requested to carry out checks on dozens of such individuals.

Mr Downey had a "letter of assurance" that he was not wanted by police and it was a crucial factor in his defence.

Mr Robinson called for all letters sent out to be rescinded and wants "full disclosure" of what had happened.  

He said: "I am not prepared to be kept in the dark by Her Majesty's government about matters relevant to Northern Ireland.

"I want a full judicial inquiry to find out who knew, when they knew and what they knew. I want to know who they are and what crimes they are believed to have committed."

Mr Robinson felt deceived by the government, and if he and former DUP leader Ian Paisley had known about this, they would not have entered into power-sharing government with Sinn Féin in 2007.

British Prime Minister David Cameron condemned the actions that led to the collapse of the case.

He said the letter giving Mr Downey a false assurance that he was not wanted by British police over the IRA attack should never have been sent.

Mr Cameron added that a rapid factual review would be carried out to make sure "this cannot happen again".

Responsibility for justice and policing in Northern Ireland in 2007 fell to Westminster and not Stormont, so the Secretary of State for Northern Ireland was the key political figure.

Sinn Féin was pressing for clarity on the status of people who at some stage during the Troubles had been linked to criminal investigations but not formally charged.

A key issue in the controversy is how and why PSNI assessments that a person was not sought in Northern Ireland were extended to include police forces beyond Northern Ireland.

Mr Robinson claimed Mr Downey had been handed a "get out of jail free card" by former prime minister Tony Blair's government.

He said justice should not have a "sell-by date".

"This conclusion is an outrage and a dark day for justice in the United Kingdom," he said.

"It is little wonder that some have lost faith in our justice system.

"Mr Downey was being tried for one of the most heinous atrocities of the Troubles, but has now invoked a get-out-of-jail free card which he and his cohorts were handed by Tony Blair's government."

Former secretary of state for Northern Ireland Peter Hain said yesterday that while he understood the anger of the families, the arrangement for dealing with the on-the-runs had been an essential part of the peace process.

"You often get this at the end of wars and conflicts. You often get what seem to be unseemly processes in order to end the violence and stop them happening again," he told the BBC.

"Awful atrocities like this hideous attack on London could happen and would still have been happening had there not been put in train - both before Tony Blair became prime minister and then at the Good Friday Agreement - a process that ended the horror, the war, the terrorism and brought Northern Ireland to where we are now with old enemies sharing power."

Mr Hain, who said that he was no longer in the post when the letter was sent to Mr Downey assuring him that he was no longer at risk of prosecution, nevertheless said he had been "astonished" by what had happened in this particular case.

"I am surprised about the detail of this case because in other cases, of which there were many, they [the PSNI] had conducted a painstaking investigation to check carefully and methodically whether it was possible to find the evidence to bring a prosecution," he said.

"They decided in Mr Downey's case that there wasn't. He was a suspect for this crime and a suspect alone. It seems that there was an error."