The Royal Society said the amount of separated plutonium, most of which is the by-product of reprocessed spent fuel from nuclear power stations, has almost doubled in the last 10 years to more than 100 tonnes.

But the eminent body, which was founded in 1660 and whose previous members include Isaac Newton and Charles Darwin, said the government lacked any coherent strategy for either its long-term use or disposal.

And it warned that the consequences could be catastrophic if any of the deadly substance was stolen by extremists wanting to create a crude nuclear device or "dirty bomb" or if there was an accident.

"Just over six kilogrammes of plutonium was used in the bomb which devastated Nagasaki and the UK has many thousands of times that amount," said Professor Geoffrey Boulton, who chaired a working group on the issue.

"We must take measures to ensure that this very dangerous material does not fall into the wrong hands."

Boulton said doing nothing was not an option, particularly as the society raised similar concerns about the security risks nine years ago. Instead, the stockpile has grown as proliferation and extremist threats have increased.

The society recommended converting the plutonium into Mixed Oxide (MOX), which it said was the most stable and secure form of spent nuclear fuel, and use it to power nuclear reactors for electricity.

Doing so would make it more difficult to steal because spent fuel is more radiocative, harder to handle than plutonium and more difficult to use in nuclear weapons because it would need to be reprocessed first.

The society said the entire stockpile could be burnt as MOX fuel if the government decides to build a new generation of nuclear power stations to meet the country's future energy needs.

If it does not, the Sizewell B facility on the east English coast could be modified to burn MOX fuel with the remainder stored as MOX fuel pellets, which would make the plutonium more secure than at the moment.

The best method of disposing of Britain's separated plutonium stockpile is burial deep underground in the form of spent fuel, or less ideally MOX pellets.

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