The unexpected suggestion by John Larkin has whipped up controversy, with relatives of those killed in the conflict expressing anger and outrage.
The chief legal adviser to the Stormont executive said he also favoured ruling out further inquests and other state investigations into the crimes committed during the 30-year conflict, insisting a line should be drawn on offences perpetrated before the 1998 Good Friday peace agreement.
Mr Cameron insisted the British government had no plans to legislate on any form of amnesty.
“The words of the Northern Ireland attorney general are very much his own words,” said Mr Cameron.
Taoiseach Enda Kenny said it would be difficult for families to accept such a move, especially if incontrovertible DNA evidence linking an individual to a killing was to emerge.
“Families want closure, but there’s always that yearning to find out what happened, who gave the instructions, why was this done?” he said.
Mr Larkin has claimed his proposals do not amount to an amnesty, but has been challenged to explain his rationale, given that perpetrators would not face justice.
Stephen Gault, whose father Samuel was killed in the 1987 IRA bombing in Enniskillen, Co Fermanagh, — for which nobody has ever been convicted — said he was disgusted by Mr Larkin’s statement.
“How dare he airbrush the innocent people who were murdered at the hands of terrorists to move things forward,” said Mr Gault. “I think it’s totally disgusting.”
Former US diplomat Richard Haass is trying to achieve political consensus on a number of issues as yet unresolved during the peace process — one of which is how the North addresses unanswered questions over killings by all sides.
Mr Larkin has outlined his proposals in a submission to Dr Haass.
Mr Cameron said it would be “rather dangerous” to block possible future prosecutions.
“I do think it’s important to allow Richard Haass to do his work about parades, about flags, and about dealing with the past,” he said.
“Clearly, the dealing with the past part is the most difficult of the three and the most difficult to unlock.”
Kate Nash, whose brother William was killed by British soldiers on Bloody Sunday in Derry in 1972, challenged Dr Haass to reject any talk of an amnesty.
At the City Hotel in Derry, Ms Nash made clear to the former diplomat her family’s opposition to the proposal.
Afterwards, she said: “What are they trying to do, draw a line under victims, draw a line under my brother? We are not going to let that happen.”
Mr Larkin he said he felt the time had come to halt prosecutions.
“More than 15 years have passed since the Belfast Agreement,” he said.
“There have been very few prosecutions, and every competent criminal lawyer will tell you the prospects of conviction diminish, perhaps exponentially, with each passing year, so we are in a position now where I think we have to take stock.”
Mr Larkin’s remarks have been criticised by politicians on both sides, with the Democratic Unionists, Ulster Unionists, and the nationalist Social Democratic, and Labour Party all voicing concern.
Sinn Féin president Gerry Adams did not explicitly endorse or condemn Mr Larkin’s stance but claimed there already was a de facto amnesty for state forces who carried out killings.