The disputed fate of 170kg of highly enriched uranium (HEU), which some suspected of having been sent to a UK nuclear weapons programme at Aldermaston, led at one point to Prime Minister Tony Blair being accused of misleading Parliament.

In June 1998, a report from a working group set up by Dounreay’s operator, the UK Atomic Energy Authority, first suggested 170kg of HEU could not be accounted for at the plant in the 1960s.

However, UK AEA chief executive John McKeown insisted it had all been an accountancy error and the material was not missing – “ is a case of the material never having existed”.

That same day, Mr Blair embraced this explanation of the “supposedly missing HEU” and commended it to SNP leader Alex Salmond in an ill-tempered exchange.

Mr Blair also rejected suggestions the 170kg had been sent to the UK nuclear weapons programme at Aldermaston, saying “no such material has ever been sent from Dounreay for use for UK weapons purposes”.

However, the following month, then President of the Board of Trade Margaret Beckett said at least a proportion of the 170kg had, in fact, existed, because some had been burned and some had been discharged.

She also conceded that nuclear material would have been sent to the weapons programme, something which had been categorically denied by Dounreay’s operators for decades and by successive governments.

Mr Blair’s office then insisted that when the prime minister had made his statement to Parliament, he meant only that none of the 170kg, which was not thought to exist, had gone to the weapons programme.

But now some of the disputed material has started to appear. Some has been located in more than 200 waste drums full of material produced while historic operations were inspected, assayed and repacked for safe long-term storage.

Decommissioning engineer Bob McKiddie said: “We had suspected that the historical results had under-estimated the uranium content in a number of waste items.

“The repackaging work has resulted in an overall gain in the amount of uranium declared.

“The figures from the repackaging work show that material previously considered ‘lost’ was in fact safely packaged as waste.”