VANGUARD - A Royal Navy nuclear submarine suffered a “concerning” malfunction while diving, the BBC has been told. The incident on board the unnamed Vanguard class submarine, which carries the UK’s trident nuclear missiles, happened more than a year ago while it was preparing to go out on patrol. A defence source said “there was a problem and it was concerning”, but that it was picked up by “the system”. A Royal Navy investigation into the incident took place, the source said. But they declined to give further details. During the incident the main depth gauge on the submarine failed as it was diving, a defence source confirmed, but a secondary depth gauge was still working. Submarines have to be strong enough to withstand enormous pressures exerted by the ocean when diving. Redundancy systems are built into nuclear armed submarines to prevent problems from escalating. Details of the incident were first reported in the Sun newspaper which said engineers on board alerted the crew to the failure while the submarine was diving.