A new investigation must be set up to examine whether the state withheld vital intelligence from detectives hunting the Omagh bombers, a parliamentary report has said.

The Northern Ireland Affairs Committee said too many questions remained unanswered over how much the security services knew about the killers' movements around the time of the dissident republican attack 12 years ago and if police officers were left out of the loop.

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 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 Royal Ulster Constabulary 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.

"Far too many questions remain unanswered," he said. "The criminal justice system has failed to bring to justice those responsible for the Omagh bombing. "The least that those who were bereaved or injured have the right to expect are answers to those questions."

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.

After reviewing the edited summary, committee members agreed with Sir Peter's claim that information obtained by GCHQ was not monitored in "real time" and therefore could not have prevented the bombing. But it raised concerns about the data flow after the attack, in particular whether names of the suspected bombers were known and, if so, why they were not passed to police officers.

In particular the inquiry said 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 RUC colleagues in the Crime Investigation Department (CID) who were working on the Omagh case.