About 30 kg of plutonium, enough to make seven or eight nuclear bombs, has disappeared from Britain's largest nuclear plant at Sellafield, a media report said on Thursday.
The annual audit of nuclear material at all of Britain's civil nuclear plants is expected to reveal that the quantity of plutonium at Sellafield was classified as "material unaccounted for" last year, The Times said.
British Nuclear Fuels, which operates the plant, has described the revelation as an "accounting issue".
"There is always a discrepancy between the physical inventory and book inventory," a British Nuclear Group spokesperson said.
Figures published by the BNG each year reveal an audit of nuclear material, which is admitted and processed by the various plants around Britain.
The spokesperson said the most likely reason for any shortfall was due to the complex measuring processes that are carried out.
Asked if the 30 kg figure raised concern, she replied, "I wouldn't say we would be alarmed by it, because we are only talking about a book figure here."
Guidelines issued by the International Atomic Energy Agency say that material unaccounted for must not exceed three per cent of the amount that is processed. If the 30 kg figure is accurate, it would equate to around 0.1 per cent of that amount, the spokeswoman said.