Sellafield has "lost" 30 kilograms of plutonium, according to figures due to be published today.

An annual audit of nuclear material is expected to reveal that the plutonium, which is enough for seven or eight nuclear bombs, was classified as "material unaccounted for" last year.

Dr Frank Barnaby, a specialist in nuclear weapons, said: "There will always be some material unaccounted for but this is a dramatic development.

"This is a major reason for not reprocessing spent nuclear fuel because you can't tell what the material unaccounted for is."