A Royal Navy nuclear sub was sinking towards its crush depth when it was saved just moments from disaster. A total of 140 crew members and Trident 2 missiles were onboard the decades-old Vanguard class vessel as it travelled through the Atlantic.

But the depth gauge, which tells sailors how far below the waves they are, failed. If they had reached the depth, it would have sparked carnage and a potentially chaotic mission to recover the top-secret vessel and its nuclear reactor.

The submarine reportedly was due to go on a patrol when the dials stopped working. Commanders are said to have believed the ship was level but it was actually still diving towards the sea floor.

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Engineers at the other side of the 500ft vessel noticed a second dial and quickly raised the alarm while the sub plummeted towards the 'danger zone', MirrorOnline reports. Speaking to The Sun, a source said: "It’s not the engineers’ job to control the sub’s depth but they saw how deep they were and realised something was wrong.

"Technically the sub was still at a depth where we know it can operate, but if it ever has to go that deep the whole crew is piped to action-stations. That hadn’t happened. The sub wasn’t supposed to be there, and it was still diving. And if it had carried on going, it doesn’t really bear thinking about."

The sub or the depths involved have not been named for security reasons. But as disaster was averted, it indicates safety systems aboard the Vanguard-class vessel are functioning properly.

An urgent safety probe has been launched but sources say the near-miss did not interrupt the UK's nuclear deterrent. Since 1969, at least one Royal Navy submarine has been patrolling the oceans to strike back should an attack be launched against UK interests or the British mainland.

Currently, the UK has four Vanguard-class submarines. Just two of them are operational as one is being renovated and the other is on sea trials.

The latter ship recently had repairs costing £300million over budget. The Navy said: "We do not comment on operations. Our submarines continue to be deployed globally, protecting national interests."