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.

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.

The paper said the submarine was still within its limits for operating safely but was diving towards its "crush depth" before the alarm was raised.

The Ministry of Defence (MoD) does not normally comment on the operations of the UK's submarines.

The Royal Navy said its submarines continue to meet their commitments and that while it does not comment on operations, the safety of its personnel is always the highest priority.

The Vanguard submarines are based at Faslane in Scotland and, according to the Royal Navy, normally have a crew of 132 officers and men.

In January this year it was reported that inspectors found a "defect" on HMS Vanguard, the lead boat of the four Vanguard-class submarines, while it was undergoing maintenance work.

The defect, from "work done in the past" was "promptly reported and fixed", the MoD said at the time.

In May the vessel finally left the Devonport dockyard where it had undergone a seven-and-a-half-year refit.