MPs who have held an inquiry into the security services' role in the Omagh bombing have said they are "bitterly disappointed" with Gordon Brown.
The Northern Ireland Affairs Committee has been investigating claims in a BBC Panorama programme that the Government's top secret listening station, GCHQ, monitored suspects' mobile phone calls as they drove to Omagh from the Irish Republic on the day of the atrocity.
Panorama said this information was never passed to Royal Ulster Constabulary detectives assigned to the case.
Twenty-nine people, including a mother pregnant with twins, were killed when the Real IRA car bomb exploded in the town.
Many of Panorama's claims were rejected in a review, completed last year, by the Intelligence Services Commissioner Sir Peter Gibson.
After analysing the summary of the Gibson report, 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 the MPs, including committee chairman Sir Patrick Cormack, were denied access to the full report, which has been classified for security reasons.
Their report says: "We repeat our bitter disappointment that in spite of repeated requests, the Prime Minister has refused to allow our chairman to read the full report, even under supervision.
"It is thoroughly reprehensible that the Government should seek to prevent the parliamentary committee charged with the oversight of Northern Ireland affairs such access."
The committee said a new investigation was necessary because the Prime Minister's request for Sir Peter to conduct a "speedy" review meant crucial questions were left unanswered.
Mr Brown's spokesman said all proper procedures for sharing intelligence on the Omagh bombing had been followed.
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.