The protesters forced their way into Dundonald House

Up to 30 protesters forced their way past the reception area at Dundonald House in Stormont on Wednesday and made their way to an office on the sixth floor, according to the Prison Service.

Staff called the police, but the group were able to get access to Prison Service offices before they arrived.

They were protesting over republican prisoners having to share cells with loyalist inmates.

Ulster Unionist David McNarry said he had asked the Security Minister Jane Kennedy to explain how one of the most sensitive government buildings in the province was occupied for almost an hour.

On Friday, Ian Paisley junior of the DUP said he was shocked at the incident.

The fact that the people were able to get into this building is a complete and total security shambles Ian Paisley Jnr DUP

"The fact that the people were able to get into this building is a complete and total security shambles.

"And we demand a statement from the secretary of state as to what they were doing.

"This is not the first time - it is getting more bizarre as each day passes. Castlereagh, Stormont, the Royal Victoria Hospital, now this, where's next?," he said.

The Prison Service has said that a review of security is under way at the site.

A spokesman for the service said that thanks to the "courage and determination of staff", all confidential filing cabinets were secured.

Details on hundreds of prison staff were found during police raids in north and west Belfast last October, which were linked to IRA intelligence gathering.

Files found by the police were understood to contain the names and home addresses of almost all 2,000 prison service staff.