The security operation which uncovered alleged republican intelligence gathering in government in Northern Ireland was triggered by a police source within the IRA, the BBC has learned.

It is now known that long before last month's raids on the Northern Ireland Assembly and elsewhere, the police, MI5 and the Northern Ireland Office already knew the IRA had obtained sensitive government documents as well as the personal details of hundreds of prison officers.

According to a senior security source the main target of the police operation was the IRA's so-called director of intelligence - a prominent west Belfast republican.

Operation Torsion - as it was called - was a follow-on investigation after the robbery at Belfast's police headquarters at Castlereagh in March this year.

John Reid: Knew the IRA had infiltrated the Northern Ireland Office

It is understood the investigation was prompted by information which came from a police source inside the IRA.

During the summer, warrants for a major bugging and surveillance operation targeted at the republican group were signed by the then Secretary of State, John Reid.

Around the same time he issued a final warning to the IRA about its activities.

As he spoke in parliament, John Reid knew that the IRA had infiltrated the Northern Ireland Office and he had a detailed knowledge on what Operation Torsion was all about.

In a major undercover investigation, the special branch sought and was given assistance by the national intelligence service, MI5.

Inquiry call

And, long before the arrests and charges came in October, it was known that the IRA had obtained personal details on hundreds of prison officers.

An assessment was made that they were under no threat from the IRA and Operation Torsion was allowed to run.

Earlier on Tuesday, a senior Ulster Unionist called for a public inquiry into allegations of IRA intelligence gathering at Stormont.

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Former culture minister Michael McGimpsey said he believed the spy ring had "penetrated to the very heart of the security situation in the Northern Ireland Office".

Mr McGimpsey made the comments after Acting Deputy Chief Constable Alan McQuillan said on Monday that the police had broken up a major IRA intelligence gathering operation in Belfast during their investigation.

Four people, including Denis Donaldson, head of Sinn Fein's administration at the Northern Ireland Assembly, were charged last month after police seized documents in raids on republican homes.

A civil servant who had access to the offices of the first and deputy first minister was arrested last week, but was released without charge. He has been suspended on full pay.

However, Sinn Fein chairman Mitchel McLaughlin said while people were facing charges, none of the allegations had been proven.

He said the important point was that Sinn Fein's project was a political one and while the row over the allegations continued "politics is in paralysis and that it not good for anyone".

'Not offensive'

On Monday, the acting deputy chief constable said the breakthrough followed the investigation into the break-in at Belfast's police headquarters in Castlereagh in March.

Mr McQuillan said the investigation had taken the police "into the very heart of the Provisional IRA".

Thousands of documents and hundreds of computer disks are being examined by 40 detectives who are working on the Castlereagh case and the alleged IRA intelligence gathering operation at Stormont.

Mr McQuillan alleged some of them had been copied in the Northern Ireland Office, while some had been "originated within the IRA".

But he said he had nothing to suggest the IRA had any intention to use the information from Stormont in any offensive way.