LONDON (AFP) - Northern Ireland's First Minister Peter Robinson threatened to resign Wednesday unless there is a judicial inquiry into the aborted trial of a man suspected in a 1982 IRA bombing in London.

Mr Robinson said the British government had kept him in the dark on the issue of secret amnesty letters given to fugitive suspects - the issue that caused the high-profile Hyde Park bombing trial to be abandoned.

A judge on Tuesday ruled that John Downey, 62, should not be prosecuted for the attack after it emerged he had received an official letter in 2007 assuring him he would not face prosecution if he re-entered the United Kingdom.

The letters were part of the 1998 Good Friday agreement that ended decades of sectarian violence in Northern Ireland.

Mr Downey was charged with killing four cavalry soldiers in the Hyde Park bombing by the Irish Republican Army. Seven military bandsmen were also killed in a separate bombing on the same day on July 20, 1982 in London's Regent's Park.

Mr Downey was arrested last May at London Gatwick Airport while en route to Greece. A visibly furious Robinson, who has headed Northern Ireland's devolved administration since 2008, said he was "incandescent with rage" that he did not know about the letters.

"I am not prepared to be the first minister of a government that is kept in the dark" by London on matters relating to Northern Ireland, he told BBC television.

"I want a full judicial inquiry into all of these matters so that we can see who knew, when they knew, what they knew. I want all of the letters rescinded," he added.

British Prime Minister David Cameron told parliament it was "absolutely shocking" that a "despicable mistake" meant Mr Downey would not face trial.

The government's chief legal adviser, Dominic Grieve, England's attorney general, told parliament Mr Downey should never have received the letter.

Such letters of assurance have been sent out to 187 so-called "on-the-runs", living outside British jurisdictions, who sought clarification on their status following the 1998 Northern Irish peace accords between its broadly Protestant British and Catholic Irish communities.

Mr Robinson, whose Democratic Unionist Party is rooted in the Protestant community, said he had been working to ensure that victims of Northern Irish terror attacks got justice.

"Now we find out that they never would get justice because there are people going round with letters stuffed in their pockets which say that even if somebody fingers you, you're not going to jail," he said. "We were lied to."

Mr Robinson said his party would never have entered into a power-sharing government in 2007 with Sinn Fein - the IRA's political wing, drawn from the Catholic community - if it had known about the letters.

Mr Cameron told parliament there would be a swift factual review into the situation but stopped short of calling for a judicial review.

He said he understood the "depth of anger and concern that people will feel" about the bombing and the "absolutely shocking" fact that Mr Downey would not now face trial.

"We should be absolutely clear: The man should never have received the letter that he received," Mr Cameron said. "It was a dreadful mistake and a mistake that we now need to have a rapid factual review to make sure this cannot happen again."