Nuclear bombs supplied to the RAF in the 1950s could have exploded by accident in test flights because they contained so much fissionable material, according to a report in New Scientist magazine today. Up to a dozen of the weapons, based on a design codenamed Violet Club, were supplied to RAF bases including Finningley in South Yorkshire, Scampton in Lincolnshire and Wittering in Cambridgeshire between 1958 and 1960.

Rushed into service as a stop-gap while Britain developed its own H-bomb, the gigantic devices risked going critical when armed.

Each bomb held around 70kg of uranium 235, enough to create a 500 kiloton explosion [equal to 500,000 tons of TNT]. By comparison, the weapon which devastated Hiroshima had a force of 15 kilotons.

The danger came when the weapons were armed and placed on "dummy run" flights to test operational readiness, according to New Scientist.

Each weapon, made by the Atomic Weapons Research Establishment at Aldermaston, was packed with 450kg of steel balls while on the ground to separate the sections of uranium so it could not form a critical mass. But the RAF feared there was a "risk of catastrophe" once this safety mechanism was disabled.

In a memo dated January 12 1959, a group captain wrote: "A high yield nuclear explosion would be possible if the weapon were jettisoned or in the event of a crash on return or an accident in de-bombing."

The RAF originally stored the bombs in separate buildings to minimise the risks, but later they were kept only six feet (1.8m) apart.

The bombs also took too long to arm. In the event of a nuclear attack, the RAF was meant to be able to launch a nuclear-armed aircraft within 15 minutes but the group captain's memo warned that removing the metal balls took 20 minutes. It meant that aircraft carrying Violet Club took up to 90 minutes to take off.

The evidence was uneathed from the public records office in London by Lesley Wright, a lecturer in statistics at John Moores University in Liverpool, David Wright of the University of Manchester, and Nicholas Hill, a physicist and historian.

Ms Wright suggested the unwieldy weapon was imposed on the RAF because the government wanted Britain to be one of the "big three" nuclear powers with the US and the Soviet Union.

"This is a prime example of how the pursuit of superpower status and fear overcame rational decision making."

In the paper on which the New Scientist article is based, the academics quote a Foreign Office official who underlined how nuclear capability was linked to national prestige.

"According to Sir Michael Perrin, [Ernest] Bevin [foreign secretary] said: "This won't do at all, we've got to have this ... I don't mind for myself, but I don't want any other foreign secretary of this country to be talked at or to by a secretary of state in the United States as I have just had ... We have got to have this thing over here whatever it costs.

"Sir Michael remembers Bevin adding 'We've got to have the bloody union jack flying on top of it'."

An MoD spokesman said the Violet Club bombs were supplied as a kit for Vulcan bombers "which could be assembled and deployed in a national emergency". The MoD insisted there was no risk of an accidental explosion because all necessary precautions were taken in transport and storage.