Tony Blair has apologised for a secret letter that let the man suspected of the 1982 Hyde Park bombing walk free from court.
Making his first public appearance in parliament since he left office seven years ago, the former Prime Minister said he accepted "full responsibility".
But he refused to say sorry for the policy to issue the letters, dubbed by critics an "amnesty" for the IRA, to almost 200 terror suspects.
Without the so-called "comfort letters", he said, the peace process could have collapsed, tipping Northern Ireland back into violence.
He told MPs : 'I accept full responsibility... for not having put in place the structure for this procedure that might have meant in the (Hyde Park) case... that the letter would not have been sent and therefore the trial would have proceeded.
'I am sorry for those people and I apologise to those people who have suffered as a result of that.
"But I am not going to apologise for sending those letters to those who should have received those letters, because without having done that, we would not have a Northern Ireland peace process.'
The letters were part of a deal between Sinn Fein and the last Labour government after the 1998 Good Friday Agreement.
But their existence was made public only recently with the collapse of the case against John Downey, accused of killing four members of the Royal Household Cavalry in London's Hyde Park.
There was outrage that the documents appeared to tell 'on the runs' that they would not face prosecution.
MPs protested that this went against the spirit of the Northern Ireland peace deal, which said suspects would still be tried although they could serve less time in jail.
Mr Blair's appearance before the Commons Northern Ireland Affairs Committee came after MPs accused him of refusing to give evidence.
He told MPs that Mr Downey had received a letter by mistake.
The documents were supposed to be sent only to those who the authorities did not want to prosecute, he said.
But MPs told him he had increased the hurt felt by victims by keeping the letters hidden from the public.
A number of key members of the peace process have told the committee that they had no knowledge of the documents, including Nobel peace prize winner and then Ulster Unionist leader David Trimble.
The DUP's Ian Paisley junior also disputed Mr Blair's claim that he had acted for the good of the peace process.
Mr Paisley said that all the evidence to the committee suggested that the IRA would not have returned to violence .
Mr Blair told the committee: "I don't want to diminish the hurt that people feel.
"But I would say that the reason we were doing what we were doing was to make sure that there were not more victims."
And he issued some advice for David Cameron on how to deal with Northern Ireland.
"You inherited a peace process that worked - so be careful with it because it's fragile still," he said.
He also said he would not have initiated the coalition government-ordered checks on the current recipients of letters.
He received support from one Labour MP Stephen Hepburn who told him "risks had to be taken to get a better society" and "thanks to you Northern Ireland has a better future".
Earlier Mr Blair had offered his support to current Labour leader Ed Miliband, suggesting critics had got it wrong when they claimed that his successor struggled to appeal to voters.
"I'm not sure he has got a problem," Mr Blair said.
"That will be for the people to choose and, as I've said all the way through - and this is to take it from the philosophical discussion about democracy down to the next election - but I'm in the Labour party and I'll be backing him."
He also described Mr Miliband's brother David, who quit frontline politics when he lost the Labour leadership battle, as a "person of very strong convictions".
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