THIRTY fires have broken out on Britain's nuclear submarines over the past three years, the Ministry of Defence has admitted.

That's an average of one every 40 days, and brings the total number of fires since 1987 to 266 – more than 10 a year. The MoD has also revealed for the first time that 74 of the fires erupted on submarines usually armed with nuclear warheads.

The disclosures have prompted accusations that Britain's nuclear weapons can no longer be regarded as a "credible deterrent". Fires could too easily knock out the submarines needed to keep patrolling, according to the SNP's defence spokesman, Angus Robertson MP.

"Any one of these fires could have had catastrophic consequences and the frequency of these incidents raises the most serious safety concerns," he told the Sunday Herald. "That so many of these incidents occurred on submarines that may have been nuclear-armed is deeply troubling."

One of the fires categorised by the MoD as major took place on a submarine capable of carrying nuclear weapons while it was docked at an unnamed naval base. Assuming this occurred in the UK, it was most likely to have been at Faslane on the Clyde or at Devonport in Plymouth on the south coast of England.

Robertson demanded to know when and where this fire took place. "The apparent vulnerability to fire events on these vessels raises grave questions for UK ministers," he argued.

"It makes a mockery of any UK claims to having a credible 'independent' nuclear deterrent."

The MoD operates four nuclear-powered Vanguard-class submarines capable of being armed with nuclear Trident missiles.

Of the 266 fires since 1987, 243 were classed as "small", such as "a minor electrical fault creating smoke". Twenty were rated as "medium" fires, "such as a failure of mechanical equipment creating smoke and flame, requiring use of significant onboard resources". There were three major fires that took place "while the submarines involved were in naval bases, requiring both ship and external resources".

In response to questions from Robertston, the UK Defence Minister, Peter Luff, said 74 of the fires were on submarines capable of carrying nuclear-armed missiles, though he declined to say whether they were armed when the fires broke out. Of those, 67 were classed as small, six as medium and one as major.

It is understood that of the 20 medium fires, only five took place at sea. "No fire onboard any Royal Navy submarine has ever had an impact on nuclear safety or the ability to operate a continuous at-sea deterrent," said an MoD spokesman.

"Meticulous records are kept of all incidents involving fire, however small. Most of those recorded were minor electrical faults that were dealt with quickly, safely and effectively. The Royal Navy operates a stringent safety regime on board all its submarines and all personnel receive regular and extensive fire safety training."