With up to 140 crew members and an unknown number of weapons, the consequences could have been dire, ITV News Political Correspondent Carl Dinnen reports
A Royal Navy nuclear-armed submarine has suffered a worrying equipment failure which could have had dire consequences.
As first reported in The Sun, the Vanguard class sub was at sea and diving when a depth gauge incorrectly told the control room crew they had levelled off in the water.
This was potentially catastrophic as the still diving submarine could eventually have reached a point where the water pressure crushed the vessel, killing the crew.
Disaster was averted by a backup system in the engineering department which correctly alerted the team there to the continuing dive.
In one sense that is exactly what is supposed to happen; if one system fails, another kicks in.
The question is how close to disaster the sub came, with up to 140 crew and an unknown number of nuclear weapons onboard.
The location and operations of the nuclear deterrent are a very closely guarded secret and even in parliament today MPs were given a firm "no comment" when they asked ministers about what had happened.
But I'm told that following an internal report into the incident there was "concern" about the depth the submarine had reached before the discrepancy was picked up.
Inevitably there are now questions about the reliability of the Vanguard class subs, which are nearing the end of their intended service lives.
Launched in the mid-90s they were supposed to start retiring five years ago but instead will now keep going until the middle of the next decade.
This means long periods in maintenance between patrols, which in turn means longer patrols for the boats which are ready.
In September a Vanguard sub set a new record for the longest continuous patrol.
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