* Demands official inquiry into secret assurances
* Issue came to light after murder accused walked free
By Ian Graham
BELFAST, Feb 26 (Reuters) - Northern Ireland's most senior politician threatened to resign on Wednesday unless there was a full inquiry into guarantees of immunity given by British authorities to paramilitary suspects as part of peace agreements for the province.
The issue came to light when an Irishman accused in a 1982 car bombing in London's Hyde Park that killed four soldiers on ceremonial duty walked free on a court finding that he had been given assurances he would not be prosecuted.
A spokesman for British Prime Minister David Cameron said he believed about 200 individuals had been guaranteed immunity and police were urgently checking if mistakes had been made in granting immunity to others.
Peter Robinson, first minister of the British province where a 1998 peace deal largely ended decades of violence, said he was not prepared to continue in the role if he was kept in the dark by London.
"This is a deliberate attempt to circumvent the courts, to allow people who are responsible for heinous crimes to come back into Northern Ireland without any fear of there being any judicial process attached to them," Robinson said in an interview with local broadcaster UTV.
"I'm not prepared to remain as First Minister of an administration that is kept in the dark, who were being deceived by government - and I'm talking about past and present."
John Downey, 62, from County Donegal in Ireland, was charged with murdering four members of the Royal Household Cavalry killed when a car bomb exploded in Hyde Park as they paraded towards Buckingham Palace. He denied the charges.
The Irish Republican Army claimed responsibility for the double bombing, one of its most devastating attacks on the British mainland during the a guerrilla campaign to drive British forces out of the province of Northern Ireland.
Downey's defence had maintained the trial should not go ahead because he had received a letter of assurance from the Northern Ireland Office in 2007 that he was not wanted in London over the bombing. The judge ruled that the assurance had to stand even if made by mistake, as it had misled the defendant. A trial would be an abuse of executive power.
The IRA abandoned its armed struggle for an end to British control of Northern Ireland and unification with Ireland in the 1998 peace deal. More than 3,600 people died, including more than 1,000 members of the British security forces, during a sectarian conflict that began in the late 1960s.
"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," Robinson said in a second interview with the BBC. (Additional reporting by Andrew Osborn in London; Writing by Sam Cage; editing by Ralph Boulton)