Former British prime minister Tony Blair has defended granting hundreds of letters to republicans telling them they were not wanted for prosecution, saying it helped stave off the collapse of the Northern Ireland peace process three times.

"This was absolutely critical to the peace process and at times was fundamental to it," he told the House of Commons Northern Ireland Affairs Committee, which had summonsed him to give evidence.

The letters became controversial after the prosecution of one recipient, John Downey, for the 1982 Hyde Park bombing, was stayed when a judge ruled he could not be tried because he had been wrongly told he was not wanted.

During a confident performance, Mr Blair questioned Northern Ireland secretary of state Theresa Villiers’s decision to declare the letters worthless. He said he would not have done so in the wake of the Downey controversy if he was in power.

Despite charges that he had bowed to Sinn Féin pressure, Mr Blair said the party had wanted much more and repeatedly complained he had not honoured the 2001 Weston Park on the runs agreement.

He said the letters had helped keep the peace process alive, particularly in late 2006 when he had believed “we had lost the whole thing” because of Sinn Fein’s refusal to support the police and because of DUP objections to being in government with Sinn Féin.