A huge crane poses the biggest risk of a nuclear disaster at the Faslane naval base on the Clyde, according to newly released safety assessments by the Ministry of Defence.

Plutonium from up to 48 nuclear warheads could escape and cause widespread contamination and cancers if there was an accident while a Trident submarine was being moved by the crane - known as a shiplift' - the reports say.

But the MoD has been accused by experts and anti-nuclear campaigners of playing down the real dangers. The amounts and risks of the radioactivity that could be released have been underestimated, they say.

The shiplift at Faslane is a unique facility with a chequered history. Set up in 1993, it uses nearly 100 winches to hoist the 16,000-tonne Vanguard-class submarines into the air for maintenance while they remain loaded with up to 48 Trident nuclear warheads.

The shiplift had to be modified in 1997, and in 2003 a report by consultants suggested accident risks had been underestimated.

Regarded by some as Faslane's most hazardous operation, there have been hints it may end up being replaced by the kind of dry dock used elsewhere.

But for now, it is still in regular use, for example lifting up the damaged HMS Vanguard after its collision with a French nuclear submarine in the Atlantic in February. Its use remains controversial.

Last week, more than 20 months after they were first requested under freedom of information laws, the MoD released two internal assessments of the accident risks posed by the shiplift. They consider the dangers of fires, explosions, the collapse of the shiplift platform or crane and even plane crashes.

They assume that, in a worst case scenario, the plutonium from all the warheads on the submarine will be released. In a fire, this could result in particles of plutonium being blown over a large area, increasing the risk of cancer for anyone who breathes them in.

The biggest risk is "societal contamination", according to a report written in 2000 by expert scientists from the Atomic Weapons Establishment at Aldermaston, Berkshire. However, the numbers indicating how far the contamination would spread, how many cancers it might cause and how probable it might be have all been blacked out.

The risks are "close to the tolerability criterion level" set internally by the MoD, the report said.

"There is a strong argument for ensuring the risks are as low as reasonably practicable. These conclusions do not constitute Atomic Weapons Establishment endorsement of the facility and procedures assessed."

John Large, an independent expert on nuclear submarine accidents, warned plutonium from a fire at Faslane could contaminate Glasgow, Edinburgh, Aberdeen and Inverness. It would depend on which way the wind was blowing, he said.

"If the containment of a nuclear weapon was breached, the consequences could be dire indeed, particularly if the plutonium was lofted high into the air by a fire," he said. "No civil contingency plans could cope with it."

Mr Large disputed the MoD's safety assessments and standards, pointing out they were agreed internally without independent oversight.

"The risks are not minimised to an acceptable level," he said.

The Scottish Campaign for Nuclear Disarmament (CND), which originally requested the assessments, argued they were "flawed". John Ainslie, CND co-ordinator, said the risks were "totally unacceptable".

He added: "Gordon Brown should be considering how to remove this risk from the Clyde, not trying to persuade us to live with it for the next 50 years."

The MoD insisted the shiplift met all its safety requirements.

"The Ministry of Defence works to ensure all risks are fully identified and adequately managed," said a spokesman. "Safety of workers at the base, the safety of the local population and the protection of the environment remain paramount at all times."