Submarines at serious risk over tests blunder

"Test blanks" were fitted to each vessel's hull valves when the submarine hull was air pressurised during on-land maintenance to test for leaks.

These were meant to be removed following the work, but were left blocked off.

This left both nuclear hunter killers without over-pressure protection on the steam generators, a leaked memo on the incident, written last month, revealed.

HMS Turbulent was operating for more than two years before the fault was discovered, while HMS Tireless, based in Devonport, Plymouth, operated for more than a year.

The document described it as a "serious incident", adding: "procedures were weak and ambiguous".

John Large, an independent consultant and nuclear engineer, said that if each vessel's reactor had experienced excess pressure it would not have been able to vent, leading to the risk of radioactive water bursting into the manned parts of the submarine.

He said: "This was a serious incident. It illustrates the breakdown of the rigid procedures required to ensure submarines are fit for service."

An MOD spokesman said: "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."