A Government inquiry into intelligence files linked to the Omagh bomb should be accelerated so it can be used in court, grieving relatives said.
Prime Minister Gordon Brown announced a review of intelligence material about the 1998 Real IRA blast after claims in a TV documentary that the bombers' phones were being tapped.
Sir Peter Gibson, the Intelligence Services Commissioner, will examine all intercept material from the security and intelligence services and how it was shared.
A BBC documentary claimed Government Communications Headquarters (GCHQ) was recording the bombers' calls before the atrocity, which killed 29 people.
Michael Gallagher, whose son Aidan died in the blast, urged the authorities to act quickly.
"If enough resources were applied to this enterprise, we don't see why this could not be done in a matter of weeks rather than months," he said.
Victims' relatives have warned the Government it faces legal action within a week if it does not hand over intelligence files which could have helped to catch the bombers.
They want to use any transcripts or recollections from individuals relating to mobile phone calls between the bombers on their way to Omagh in their ongoing civil legal case in Belfast against five men they blame for the tragedy.
Nobody has been convicted of murder after the Northern Ireland conflict's worst atrocity in August 1998 and angry families are demanding an independent cross-border public inquiry into events before the blast and the security force investigations.
Mr Gallagher added: "We welcome the swiftness with which the Government has moved on this. However, we believe that the families should have an input and should be consulted. It is indicated that this will take up to three months. We would urge Sir Peter to seriously consider making this material and evidence available as soon as possible."
