Russia has admitted that customs officials thwarted more than 120 attempts to smuggle "highly radioactive" material out of the country last year.

The disclosure is likely to fuel concern about how many illegal exports were not halted. It will also lead to new fears that Moscow has failed to stop material becoming available on the black market that could be used by terrorists to make radioactive "dirty" bombs.

A further 722 cases of illegal importing of highly radio-active material into Russia were detected - possible evidence of a dangerous trade between ex-Soviet states.

The figures come as Britain continues to demand the extradition of the Russian MP Andrei Lugovoi for allegedly poisoning the former KGB officer Alexander Litvinenko with radioactive polonium-210 in London in November 2006.

British sources indicate that this polonium originated at a Russian state facility. But Nikolai Kravchenko, of the Russian customs service, said: "Industrial shipments of this substance are being exported from Russia legally."