Britain would probably need help from other countries to treat casualties in the event of a dirty bomb attack, a leading scientist said yesterday.

Professor Nigel Lightfoot, chief adviser to the head of the Health Protection Agency, said other countries had agreed to help Britain, in a reciprocal arangement.

Professor Lightfoot said that any dirty bomb - a device which combines conventional explosive with radiation - would probably cause more damage than the radiation released but the terror it would cause among the population would be "quite considerable".

He said there had been a "tremendous improvement" in the way Britain was facing such threats since 2001 but the authorities had not planned for the release of polonium-210, which was used to poison former spy Alexander Litvinenko, 43, an ex-KGB agent who had become a harsh critic of the Kremlin.

He died in hospital in London in November last year, three weeks after ingesting a large dose of the radioactive material.

More than 750 people were tested for exposure and around 130 were found to have come into contact with polonium-210.

Seventeen people had levels which might lead to an increase in their long-term risk of getting cancer. The substance was found at dozens of sites across London.

Russia refuses to extradite former KGB agent Andrei Lugovoy, who is wanted by British police over the death of Mr Litvinenko.

Professor Lightfoot said: "Quite honestly, we didn't have a polonium-210 plan. We never expected it to be a problem we would face."