Tony Blair has said the peace process in Northern Ireland would probably have collapsed without the On The Runs (OTRs) scheme for fugitive IRA members.
His Labour administration sent about 200 letters to republicans assuring them they were not being pursued by the UK authorities following requests from Sinn Fein.
An investigation was launched by MPs when the prosecution of a man for the murder of four soldiers in a bombing in Hyde Park in 1982 was halted after he received one of the letters in error.
The OTR letters scheme began while Mr Blair was premier, and the chairman of the Northern Ireland Affairs Committee has said he is one of the most important witnesses to the inquiry.
Blair told MPs investigating the scheme: "The issue of OTRs was absolutely critical to the peace process and at certain points became fundamental to it.
"If I had been saying we are not dealing with this in any way at all, you can never be certain of these things but I think it is likely that the process would have collapsed."
He said Sinn Fein would have walked away and added the scheme was central to getting Sinn Fein on board with overhauled policing arrangements.
"The purpose on everything we have done was to create peace in Northern Ireland so that there were not more victims of terrorism and more families distressed and losing loved ones as a result of that terrorism."
Blair began the peace process scheme in 2000 which saw 95 of the so-called letters of comfort issued by the Government to suspects linked by intelligence to almost 300 murders.
They told people they were not wanted at that time but did not rule out future prosecutions if new evidence became available.
The scheme was drawn up following pressure from Sinn Fein to allow the fugitives, who had they been in prison before 1998 would have been released under the Good Friday Agreement, to return to Northern Ireland.
Blair said he would not apologise to those who should have received the letters. "Without having done that we would not have a Northern Ireland peace process."