International experts in radiation disasters believe that a terrorist attack with a "dirty" nuclear bomb will take place on a high-profile civilian target, such as the London Underground.

"The Tube in London for instance is a fantastic dispersion device. When the train is coming it is like a piston. You just open the canister and I would say that after two or three hours you'll have caesium all over the Tube in London," a senior diplomat and nuclear scientist tells today's Independent.

"Nothing will happen from the health point of view but people will be so afraid that no one will use it...I know the London Underground has a working party looking at this."