Two british nuclear submarines went to sea with a potentially disastrous safety defect which left both vessels at risk of a catastrophic accident, it has emerged.
Safety valves designed to release pressure from steam generators in an emergency were completely sealed off when the submarines Turbulent and Tireless left port, a leaked memo to The Guardian reveals.
The problem went undetected on Turbulent for more than two years, during which time the submarine was on regular active service. It was not noticed on Tireless for more than a year, and was finally detected last month, two months after Tireless started sea trials from its port at Devonport naval base in Plymouth.
Tireless was involved in another serious incident in 2007, when two submariners were killed in an explosion in air purification equipment.
The Ministry of Defence memo admits that both cases involving the sealed-off valve were "a serious incident", raising major questions about "weak and ambiguous" safety procedures at Devonport dockyard and within the Royal Navy.
The blocked valves, on the hull of the submarines, meant that steam from nuclear-powered boilers could not have been released in an emergency, leading to a potentially disastrous build-up of pressure.
John Large, a consultant on nuclear safety, said: "It was a very significant failure. These two submarines were unfit for service. It was a perilous situation."
An MoD spokesman said: "We can confirm that, as part of routine maintenance checks, an issue was identified on Turbulent and Tireless which has now been resolved.
"We take safety extremely seriously and as soon as we were aware of this potential issue we took action to address the problem. Detailed investigations to assess the cause and any possible safety implications are ongoing and it is too early to speculate on the outcome of those investigations."