"This is material that is unaccounted for, and there is always a discrepancy between the physical inventory and the book inventory," said a spokesman for the British Nuclear Group (BNG) which audited the plant, confirming a report in the daily The Times that some 30 kilograms (66 pounds) of plutonium could not be traced.

"There is no suggestion that any material has left the site," which is located in Sellafield, northwest England," she added. "When you have got a complicated chemical procedure, quite often material remains in the plant."

The United Kingdom Atomic Energy Authority, the country's nuclear regulatory body, said there was no reason to believe that "real loss" of plutonium had occurred.

"The material unaccounted for 2003-2004 were all within international standards of expected measurement accuracies for closing a nuclear material balance at the type of facilities concerned," it said.

"We have published these figures since the 1970s". Some years there is an apparent gain, some years there is an apparent loss," the BNG spokeswoman 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.

Guidelines issued by the International Atomic Energy Agency (IAEA) say that material unaccounted for must not exceed three percent of the amount that is processed.

If the 30 kilogram figure is accurate, it would equate to around 0.1 percent of that amount, the spokeswoman said.

A spokesman for the British Department of Trade and Industry said: "This is an account of an ongoing process and does not represent the loss of any actual material.

"It is not unusual for the accounting process to indicate material unaccounted for."

But independent experts were worried about the disclosure, according to the Times.

"They make this claim of an auditing problem but I would expect them to be overzealous in the current climate of fears about terrorism," John Large, an independent nuclear consultant, was quoted as saying.

Frank Barnaby, a specialist in nuclear weapons, told The Times "there will always be some material unaccounted for but this is a dramatic development."

Spent nuclear fuel rods, which have been inside nuclear reactors for about five years, are taken to Sellafield for reprocessing.

The Sellafield complex, which employs nearly 10,000 people, comprises a 49-year-old nuclear power station, which is currently being dismantled and decontaminated, spent fuel reprocessing facilities and a site to produce MOX, a fuel made up of a mixture of uranium and plutonium.

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