The IRA has been blamed for the multi-million pound Northern Bank raid in Belfast.
Hugh Orde met key figures in the Policing Board
Chief Constable Hugh Orde said that organisation was responsible after meeting key members of the Policing Board on Friday.
The Northern Bank has now reassessed the amount stolen from its head office on 20 December as £26.5m.
The IRA said it was not involved in the bank robbery and Sinn Fein leaders have said they believe the denial.
The Northern Bank now intends to withdraw most of its current notes and re-issue them in a different colour and style.
To date, the police have made no arrests nor have they recovered any money from the raid, thought to have been one of the UK's biggest cash robberies.
A number of homes in republican areas of Belfast and several business premises have been searched by police in recent weeks.
Mr Orde told a news conference in Belfast: "In my opinion the Provisional IRA were responsible for this crime and all main lines of inquiry currently undertaken are in that direction."
But, he said, he had not bowed to any pressure to attribute blame. He said he was doing so now because it made "operational sense".
Mr Orde also said the raid was not a victimless crime, but was "violent and brutal... not some Robin Hood effort".
The chief constable refused to be drawn on the likely political consequences of his announcement.
Afterwards, Downing Street said the prime minister had made it clear the political institutions could only be restored if there was a "complete end" to all paramilitary and criminal activity.
The spokesman confirmed that the matter will be looked at by the IMC (Independent Monitoring Commission) in due course.
Irish Prime Minister Bertie Ahern said trust and confidence in the peace process had been damaged.
"An operation of this magnitude... has obviously been planned at a stage when I was in negotiations with those that would know the leadership of the Provisional movement," he said.
However, Sinn Fein's Martin McGuinness MP said Mr Orde's comments were "nothing more than politically-biased allegations".
"This is more to do with halting the process of change which Sinn Fein has been
driving forward than with anything that happened at the Northern Bank," he said.
SDLP leader Mark Durkan said the raid had dealt a "real blow" to the Agreement and the peace process.
Democratic Unionist Party (DUP) leader Ian Paisley is to meet Prime Minister Tony Blair in Downing Street next week to discuss the ramifications of the chief constable's comments.
Mr Paisley is urging the government to move ahead and create a devolved executive without Sinn Fein.
He said that they have now "done a deed that has put them outside the present political initiative".
Ulster Unionist leader David Trimble said: "Gerry Adams has betrayed the prime minister personally.
"I call upon him to use the power created by Ulster Unionist Party pressure and exclude Sinn Fein from any Northern Ireland Assembly."