Of those five, HMS Astute, Britain’s brand new £1.2billion attack sub which ran aground off Skye in 2010, is still not fully operational, and at least one other is undergoing maintenance.



The leak will also fuel the heated political debate about nuclear submarines operating in Scottish waters. Alex Salmond has offered a cast-iron guarantee that he will expel Trident from the Clyde – where it supports up to 11,000 jobs – if Scots go for independence.



Last night, Angus Robertson MP, the SNP’s Westminster leader and defence spokesman, called on the Ministry of Defence to clarify exactly what had happened. He said: “This is the latest in a long line of alarming incidents involving nuclear submarines off the coast of Scotland.



“We need assurances from the MoD about the particular circumstances of this one, and we need to know that all necessary safety procedures were enacted fully and there was no danger to public safety.”



Royal Navy officials were unable to say exactly where the sub was when the leak developed, but sources confirmed it was in Scottish waters.



Launched in 1984, HMS Tireless was due to be decommissioned this year, but her service was extended by four years due to the delay in the roll-out of the new Astute-class submarines.



It is the second time that “HMS Tired”, as she has been dubbed, has been forced to spend almost a year out of action due to reactor problems.



In May 2000 she was stranded at Gibraltar, where she stayed for 12 months, after being forced to make an “emergency call’ at the port following a radiation leak.



It was later revealed that the submarine’s pressurised water reactor had suffered a crack in its coolant system, which could result in the uranium rods being exposed.



The latest incident comes just weeks after the Clyde-based ballistic nuclear submarine HMS Vigilant was stranded in the US after its rudder broke, despite a £350million refit.



Speaking last night former First Sea Lord Admiral Lord West said: “Clearly, it is important to be cautious when dealing with nuclear subs.



“However, it is a fact that T-class submarines are old, with operating systems designed more than 30 years ago.



“Because of problems in ordering the new Astute-class submarines, they are replacing T-Class boats later than one would have hoped. We can’t afford problems like this.”



Andy Smith of the UK National Defence Association, said: “The problems with HMS Tireless illustrate the folly of trying to have ‘defence on the cheap’ and failing to upgrade or replace equipment due to political short-sightedness and a defence policy dictated by the treasury rather than the military.”



A Royal Navy spokesman said: “HMS Tireless returned to Devonport Naval Base last week for repair following a small coolant leak that was contained within the sealed reactor compartment.



“There is no risk to the public, the environment or the crew.”