A Royal Navy nuclear sub was saved moments from disaster after a malfunction.
The Vanguard class vessel was in the Atlantic when a depth guage malfunction caused it to start to sink.
It is believed the sub was carrying 140 crew and Trident 2 missiles.
Depth dials stopped working on the vessel just as it was about to go on patrol.
The Vanguard class vessel was approaching the "danger zone"
PA
Commanders were left thinking the ship was level when in reality it was still diving.
It was only when engineers at the back of the 490ft Vanguard-class vessel spotted a second gauge showed the sub approaching the "danger zone".
“It’s not the engineers’ job to control the sub’s depth but they saw how deep they were and realised something was wrong," a source told The Sun.
"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.
LATEST DEVELOPMENTS:
The depths of the sub and the name of the sub are not being named
PA
“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 depths of the sub and the name of the sub are not being named due to security reasons.
Senior officials launched an urgent safety probe but insiders insisted the drama did not interrupt the UK’s nuclear deterrent.
Britain has four Vanguard-class submarines, however currently only two are operational.
One is currently undergoing sea trials after repairs that ran £300million over budget. Anoher is currently having a refit
The Navy said: “We do not comment on operations. Our submarines continue to be deployed globally, protecting national interests.”
At least one Royal Navy submarine with nuclear missiles has been on patrol continuously since 1969.