The following correction was printed in the Observer's For the record column, Sunday December 10 2006





An investigation was under way last night into Russia's black market trade in radioactive materials amid concern that significant quantities of polonium 210, the substance that killed former spy Alexander Litvinenko, are being stolen from poorly protected Russian nuclear sites.

As British police drew up a list of witnesses for questioning over the death, experts warned that thefts from nuclear facilities in the former Soviet Union were a major problem.

A senior source at the United Nations nuclear inspectorate, the International Atomic Energy Agency, told The Observer he had no doubt that the killing of Litvinenko was an 'organised operation' which bore all the hallmarks of a foreign intelligence agency. The expert in radioactive materials said the ability to obtain polonium 210 and the knowledge needed to use it to kill Litvinenko meant that the attack could not have been carried out by a 'lone assassin'.

Suggestions that the death may have involved some form of state sponsorship were being investigated by MI5 and MI6 who are looking at theories that foreign agents may have been behind the death of Litvinenko. Scotland Yard has asked the Kremlin for help with its inquiries, though Russia has dismissed any involvement in the death as 'absurd'. Litvinenko received British citizenship this month.

A senior British security source said they were providing the police with material in 'hostile intelligence agencies' operating in the UK, including those from Russia. He said: 'Russia has never really decreased its activity in the UK from the end of the Cold War.'

Privately, however, there is deep scepticism in Whitehall about whether the Putin administration would be willing to risk a crisis in British-Russian relations by directly authorising an assassination of a British citizen on British soil, particularly using a method that might involve other Britons being contaminated. The two countries are currently engaged in delicate negotiations over energy security.

More than anything, the death of the London-based former KGB spy has placed Russia's still thriving trade in radioactive material under scrutiny. 'From the terrorism threat standpoint, these cases are of little concern but they show security vulnerabilities at facilities,' said an IAEA spokesman.

One of the few figures available, on a database compiled by researchers at Stanford University in the US, revealed that about 40kg of weapons-usable uranium and plutonium were stolen from poorly protected nuclear facilities in the former Soviet Union between 1991 and 2002. Although the IAEA has no confirmation of polonium finding its way into the underground trade, there have been several unconfirmed reports of thefts.

In 1993 the Bulletin of Atomic Scientists reported that 10kg of polonium had disappeared from the Sarov, which produces the rare radioactive material and is described as Russia's own version of Los Alamos, the US government's nuclear research base in New Mexico.

Globally there have been more than 300 cases during the past four years where individuals have been caught trying to smuggle radioactive material. In 2005 there were 103 confirmed incidents of trafficking and other unauthorised activities involving nuclear and radioactive materials, many involving Russia.

In one incident, in the remote west of former Soviet Georgia, a group of woodsmen found two capsules of the material which was emitting heat in a forest. They used them to keep warm at night but soon developed acute radiation sickness. The capsules turned out to be the highly radioactive strontium 90 core of a nuclear generator from a long abandoned aircraft navigation beacon.

Meanwhile in Britain, Cobra, No 10's crisis committee, met again yesterday to discuss emerging findings in the police investigation and in public health.

The Foreign Office held a meeting on Friday with the Russian ambassador to request full co-operation from the Russian government in the police investigation, including making witnesses available. Officials from the Atomic Weapons Research Establishment in Aldermaston, Berkshire, and Porton Down, the government's Defence Science and Technology Laboratory, near Salisbury, Wiltshire, were trying last night to track down the precise source of the polonium 210 that killed Litvinenko.

No date has been set for a post mortem examination on Litvinenko until a risk assessment is carried out to see if it is safe to perform the procedure, and if so, what precautions would be necessary.