London: British Prime Minister David Cameron has announced 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.

Mr 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.

Freed: John Downey, accused over the IRA's deadly 1982 attack on soldiers in Hyde Park, was sent a letter assuring him that there was no police interest in him. Credit:Getty Images

Northern Ireland's most senior politician, First Minister Peter Robinson, had threatened to resign unless there was a full inquiry into the guarantees, saying they were a "deliberate attempt to circumvent the courts".

Mr 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.