Britain's Department of Trade and Industry has said the 30 kilograms of unaccounted-for plutonium at the Sellafield power station is an auditing issue and does not involve any material.
Britain's Department of Trade and Industry
A spokesman said: "It is not unusual for the accounting process to indicate material unaccounted for."
An annual check of nuclear material at all of Britain's civil nuclear installations showed that 29.6 kilograms of plutonium was unaccounted for in the last financial year at the Sellafield site in Cumbria.
The UK Atomic Energy Authority said the figure was within international standards of expected measurement accuracies.
The authority said that in most cases, unaccounted material resulted from uncertainties in the measurement systems used to produce the basic data and did not indicate real gains or losses of nuclear material.
"The MUF (material unaccounted for) figures for 2003/04 were all within international standards of expected measurement accuracies for closing a nuclear material balance at the type of facility concerned.
"There is no evidence to suggest that any of the apparent losses reported were real losses of nuclear material," the authority said.
Green Party leader Mr Trevor Sargent said he was concerned that the material could fall into the hands of terrorists.
"Plutonium is a 'hot material' which is being sought by terrorist groups across the world. There's enough (missing) material to make a number of nuclear bombs," he said.
A British Nuclear Group spokeswoman at Sellafield said: "This is material that is unaccounted for, and there is always a discrepancy between the physical inventory and the book inventory.
"There is no suggestion that any material has left the site."