Former British Prime Minister Tony Blair has said the Northern Ireland peace process would have collapsed if he had not agreed to send "letters of comfort" to IRA terror suspects.

Mr Blair has told the Northern Ireland Select Committee the controversial distribution of the letters to so-called "on the runs", stating they were no longer wanted for past crimes, were not an "amnesty" and were "not secret".

He "defended completely" agreeing to the controversial letters and said an agreement on how to treat those wanted for terrorist crimes during the troubles "was critical to the peace process" and "became fundamental".

Mr Blair told MPs that had he not agreed then Sinn Fein would have walked away from the Good Friday Agreement.

The letters from the Police Service of Northern Ireland sparked uproar last year when it resulted in the collapse of the Hyde Park bomb trial at the Old Bailey.

John Downey, who denied murdering four soldiers in the 1982 bombing, had received a letter in error informing him he was no longer being pursued in Northern Ireland or elsewhere.

During his trial, it emerged that 187 people had received similar assurances - the majority of them republicans who had never been charged or who had been convicted but then escaped.

He accepted responsibility for not putting in place a proper procedure, which led to some fugitives being mistakenly sent letters, and apologised to the people who had been affected by this.

However, he said he would not apologise for the letters sent to the people who should have got them.

Under the terms of the Good Friday Agreement, those convicted of terrorist offences were released from prison early but those who had gone "on the run" were not considered eligible.

In 2006, after an attempt to establish a formal scheme for "on the runs" failed, Mr Blair wrote to Gerry Adams outlining plans to resolve the issue, "expediting the existing administrative procedures".

The scheme had been reported publicly but victims' families said they felt "devastatingly let down" by the collapse of the Downey case - and Northern Ireland's First Minister threatened to resign.

MPs questioned Mr Blair's claim that had he not made the agreement "over on the runs" the peace process would have collapsed saying that Lord Mandelson, a former Northern Ireland Secretary, had told the committee the Good Friday Agreement was not at stake.

Mr Blair said that he agreed with the inquiry carried out by Lady Justice Hallet, which concluded that the letters were not an amnesty and that the scheme was lawful but called Mr Downey's letter a "catastrophic mistake".

But he said the letters were a way of trying to avoid more violence.