Progress is possible in the Northern Ireland process but the IRA must stop all violence, Tony Blair has said.
Tony Blair said the chief constable had evidence of IRA involvement
The prime minister was speaking after PSNI Chief Constable Hugh Orde said he believed the IRA was behind December's £26.5m Northern Bank raid in Belfast.
Sinn Fein has rejected the claim. The DUP is calling on the government to move ahead without republicans.
Mr Blair said there must be "a definitive end to all forms of paramilitary or criminal activity".
He told the BBC's Breakfast with Frost programme: "There cannot be a proper comprehensive deal for peace in Northern Ireland unless the IRA clearly and definitively give up not just terrorist acts of violence, but criminal acts of violence."
Mr Blair said the chief constable would not have made these claims without evidence.
Chief Constable Hugh Orde blamed the IRA for the raid
He said unionists were "entirely justified" to refuse to share power with Sinn Fein, "unless there is a definitive end to all forms of paramilitary or criminal activity by one of the parties that is associated with a paramilitary group".
The prime minister added: "I still think it's possible for us to make progress, but it can't be 99% giving up violence, and it certainly can't be 80% giving up violence - it has got to be 100%.
"I said that almost two years ago, and I mean it.
"If people understand that, we can make progress very quickly but there has got to be no ambiguity about it."
Democratic Unionist Party leader Ian Paisley will meet Mr Blair next week when he will call on the government to form a devolved administration without republicans.
Irish Prime Minister Bertie Ahern said he had no doubt the Provisional IRA was behind the robbery.
Speaking on RTE radio, Mr Ahern said he was convinced that during the intensive political talks before Christmas, the republican leadership would have known what was being planned.
He added: "I think ultimately we all have to be big enough to keep the peace process going forward."
"At the end we were essentially left with two outstanding difficulties which have to be resolved - one was the transparency surrounding the decommissioning of all IRA weapons and the other was IRA criminality.
"We wanted particular wording about IRA criminality.
"It was not possible for the Sinn Fein leadership to agree with that so therefore, as one of my colleagues said in those negotiations: 'If they can't agree, why is it they cannot agree'.
"And of course that leads to the inevitability of the question is: 'Was the reason because they knew these kind of events were going on?"'
Sinn Fein chairman Mitchel McLaughlin said Mr Ahern's remarks were "a direct attack on the integrity of Gerry Adams and Martin McGuinness".
Millions of pounds were stolen from the bank on 20 December
"Many nationalists and republicans will be deeply disappointed that the Taoiseach has chosen to believe the British and to jump onto the DUP bandwagon of blame," he said.
The bank raid is thought to have been one of the UK's biggest cash robberies.
The robbers stole millions from the vaults of the bank on 20 December as the families of the two bank officials were held hostage.