Allowing paramilitary fugitives to return to NI will cause a lot of pain and anguish which is going to have to be dealt with, Tony Blair has said.
Fugitives could return without going to jail
MPs are debating proposed legislation to allow fugitives to return without having to serve prison sentences.
The prime minister was responding to a question from DUP MP David Simpson.
He asked Mr Blair what the British people would think if the killers of a female police officer in Bradford last Friday were granted an amnesty.
It is thought the Conservatives and the Liberal Democrats will join unionists and the SDLP in opposing the bill in the Commons.
The proposals cover up to 150 people wanted for crimes committed before the Good Friday Agreement in 1998.
"What would the British people, or members of his party, think of the prime minister if he offered an amnesty to the murderer of the police officer?" Mr Simpson said.
"Later on today we are going to debate it in the house in regards to Northern Ireland and I would ask the prime minister to give us a comment please."
In response, Mr Blair said he understood "the pain and anguish" the legislation would cause.
"I am meeting with the RUC widows later on this afternoon and I totally understand the pain and anguish there will be," he said.
"I hope he understands that as a result of the Good Friday Agreement those people convicted of terrorist offences before 1998 have been released, it is now necessary to deal with those who have not been convicted but nonetheless have been suspected of such offences.
"That is the reason for the measures we are bringing forward. I understand the pain and anguish that it causes, but I hope that he understands this is something that has to be dealt with."
Earlier on Wednesday, NI Secretary Peter Hain denied the fugitives were getting an easy option.
"It's part of bringing closure to NI's past, just as after the Agreement over 400 paramilitary prisoners were released on licence," he said.
"If they breached the terms of these licences, as some did, they were hauled back in and this could happen to these people who are now outside the reach of the UK jurisdiction.
"This will bring them to justice, although they will not serve a prison sentence, but if they breach the terms of their licence they will be hauled in and have to serve their sentence.
The legislation will deal with people suspected of terrorism who have not been brought to court and those who have fled prison.
Sinn Fein has repeatedly pressed for them to be able to return to Northern Ireland.
If the legislation is passed, paramilitary fugitives would have their cases heard by a special tribunal, but, if found guilty, would be freed on licence without having to go to jail.