The lack of a functioning radio communications system within London's underground limited the response to the July 7 bombings last year, a new report has concluded.

Emergency workers were forced to run back and forth between trains, platforms and the surface to communicate since no mobile radio communication systems were available.

Four bombs detonated on a bus and at three tube stations killed 56 people and injured more than 700.

Near one of the bombing sites, Russell Square, a "leaky feeder" two-way antenna system used by train drivers to communicate with their line control managers was damaged in the explosion.

Mobile phones do not work in the subway tunnels and work on the aging subway system is difficult due to the narrowness of the tunnels.

The authority responsible for the Underground, Transport for London, is investing £2 billion to install a digital radio system in the tunnels, some of which date to the mid-1800s. The plan spans 20 years.

But the report, produced by the 7 July Review Committee, said the time scale is too long, and an interim system should be identified.

"Digital radio will be crucial in the event of a future emergency on the tube," it said.

By the end of 2007, London's police, fire and ambulance authorities are scheduled to complete rolling out an interoperable digital radio system, under the names Airwave and Connect. That system will also work in subway tunnels.

While responding to the bombings, the city's emergency service relied too heavily on mobile phones, leading to communication problems on jammed networks.