A NEW investigation is being called for into whether vital intelligence gathered by GCHQ in Cheltenham was withheld from detectives hunting the Omagh bombers.

An influential Parliamentary group of MPs said too many questions remained unanswered over how much the security services knew about the killers' movements around the time of the dissident republican terror attack 12 years ago, and if police officers were left out of the loop.

Members on the Northern Ireland Affairs Committee said in particular there was a need to establish the part played by RUC Special Branch – the police's anti-terrorism unit – and whether it was handed data by GCHQ but failed to pass it on to CID colleagues who were working on the Omagh case.

GCHQ has declined to comment as it had yet to examine the full report.

Twenty-nine people, including a mother pregnant with twins, were killed when the Real IRA car bomb exploded in the Co Tyrone market town.

No one has been successfully convicted of the murders, but last year four men were found liable for the bombing in a landmark civil case taken by the victims' families.

The Commons committee undertook an inquiry into the security services' role following claims in a BBC documentary that the Government's listening station GCHQ had monitored suspects' mobile phone calls as they drove to Omagh from the Irish Republic on the day of the atrocity in August 1998. Panorama said this information was never passed to RUC detectives assigned to the case.

While a subsequent review by Intelligence Services Commissioner Sir Peter Gibson rejected many of Panorama's assertions, committee chairman Sir Patrick Cormack said the bereaved still needed answers.

Sir Patrick also criticised the Government for refusing to give the committee sight of the commissioner's full report, which has been classified for security reasons.

Committee members agreed with Sir Peter's claim that information obtained by GCHQ was not monitored in 'real time' and so could not have prevented the bombing.

But it raised concerns about the flow of information after the outrage. As well as calling for a fresh examination of the intelligence, the committee's report found questions remain about whether the bombing could have been pre-empted by action against terrorists who carried out bombings in 1998.