Two policemen refused to give evidence to a high-level probe into Omagh bomb intelligence in Northern Ireland.

A member from the Royal Ulster Constabulary (RUC) Special Branch was among those who failed to co-operate with the inquiry, Sir Peter Gibson added. His investigation rejected claims information about the attack, which killed 29 people, was held back.

A Westminster committee of MPs is hearing evidence on his review, established after BBC Panorama claimed the government's communications intercept centre was listening to mobile telephones used to co-ordinate the 1998 Real IRA explosion.

Sir Peter said one of the officers who snubbed him was on the investigating team.

"The other was Special Branch, he didn't lead a team," he said.

He added: "It would have been nice confirmation for me if what I had heard was the truth, obviously one wants as much confirmation as possible."

In his report Sir Peter said details from telephone intercepts from the Government Communications Headquarters (GCHQ) were passed on promptly and fully in accordance with proper procedures.

Michael Gallagher, whose son Aiden died in the blast, said: "It is unfortunate these people that have spent a lifetime in law enforcement have decided not to co-operate.

"Why would they not want to continue to support law and order and to shed light on the matter?"

The car bomb exploded in the Co Tyrone town in August 1998 killing shoppers and sightseers.

A Panorama programme claimed intelligence officers based at GCHQ monitored the bombers' phone calls but failed or refused to pass information to RUC detectives hunting the killers in the days following the attack.

Following last September's broadcast, Prime Minister Gordon Brown commissioned Sir Peter, the Intelligence Services Commissioner, to review all intelligence material stemming from the bombing.

The Northern Ireland Affairs Committee launched an investigation following disagreements between Sir Peter and the Panorama programme makers.

Sir Peter acknowledged report maker John Ware felt he had been treated badly by him.

"I am afraid I think the BBC got it completely wrong," he said.

He added: "I am not aware of any request made by Special Branch to GCHQ that was not complied with."

He said GCHQ's role was to pass on the information to experts, in this case Special Branch.

His original report of around 60 pages has not been published and committee chairman Sir Patrick Cormack has been barred from seeing it.

A summary of 15 pages has been published but committee members were scathing at the Government's lack of disclosure.

Sir Peter said: "I would have been delighted that as many people as possible should see the full report so that they could see for themselves the extent to which chapter and verse had been provided."

Chairman Sir Patrick said: "We do find it so difficult that I have not been able to see this...we do feel disadvantaged."