A man walks past Irish Republican hoardings in the Kilwilkie estate, near Lurgan in Northern Ireland yesterday. Northern Ireland’s most senior politician threatened to resign 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.

Reuters/London

Prime Minister David Cameron announced yesterday an independent inquiry into letters of immunity sent to IRA suspects after an angry response to the freeing of an Irishman accused of a 1982 bombing that killed four soldiers in London.

Cameron said an independent judge with full access to government files and officials would lead the review and report back by the end of May.

“It is vital that we deal properly with the events of the past but make sure this never undermines our determination to build a shared and prosperous future for the next generation so we never again return to the horrors of the past,” he told reporters.

Cameron said it was clear that there had been a “dreadful mistake” in the case of John Downey, who walked free from a London court this month because of an immunity letter.

Downey, 62, from County Donegal in Ireland, was charged with murdering four members of the Royal Household Cavalry who were killed 32 years ago when a car bomb exploded in Hyde Park as they paraded towards Buckingham Palace on July 20.

The Irish Republican Army claimed responsibility for two bombings that day but Downey denied the charges. His defence argued that his 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 in connection with the bombing.

But in fact London police had never lost interest in arresting Downey and last May they detained him at Gatwick Airport en route to Greece. The presiding judge at the Old Bailey court ruled that the false assurance was a “catastrophic failure” that misled the defendant and ruled the trial would be an abuse of executive power. Cameron said it was important to find out the facts. “There was never any amnesty or guarantee of immunity for anyone and there isn’t now,” Cameron said. “It is right to get to the bottom of what happened.”