The RUC Chief Constable, Sir Ronnie Flanagan, has told a hearing at Maghaberry prison, Co Antrim, that the UDA leader, Johnny Adair, should remain in jail.
He appeared as a key witness for the Northern Secretary, Mr Peter Mandelson, who is opposing the Sentence Review Commission's preliminary decision to re-release Adair.
Lawyers for Mr Mandelson yesterday argued strongly for the UDA leader to remain in jail. The hearing continues today and a decision is not expected for several days.
Sir Ronnie spent over two hours inside the jail outlining RUC objections to any release. Several other police officers gave evidence, including at least two Special Branch members who produced an intelligence assessment of Adair's behaviour during the summer.
A senior Northern Ireland Office official said it was highly unusual for the Chief Constable to give evidence personally but it indicated the deep concern over a possible release.
Adair, the UDA's Shankill Road commander, was jailed for 16 years in 1996 for directing terrorism. He was granted early release under the terms of the Belfast Agreement, in September 1999. He was rearrested last August after Mr Mandelson decided he had breached the terms of his early release licence. Security sources believed Adair was involved in starting the loyalist feud and in raising sectarian tensions by attacking nationalist homes in north Belfast. The UDA claimed Adair's return to jail was effectively detention without trial.
The RUC believes that the loyalist leader's return to the streets would raise sectarian and inter-loyalist tensions.
Leading UVF figures are also opposing his release. The UDAUVF feud, which claimed seven lives, formally ended last month.
The hearing is taking place in Maghaberry's hospital wing. Adair is being held in the jail. His lawyers asked for a representative of the Human Rights Commission to be allowed to observe the private proceedings.
However, it is understood the application was dropped after NIO lawyers said they were prepared to go to the High Court to block the move. Adair's lawyers then told the hearing they had decided not to call any witnesses. They argued that as the commission had already found in favour of Adair in its preliminary decision, the onus was on the NIO to prove why his release should be blocked.