A submarine carrying nuclear weapons has been on patrol at all times since 1969 as part of the UK’s continuous at sea deterrent.
The Royal Navy has four Vanguard class vessels which fulfil this role on rotation. They carry around 140 crew, as well as Trident ballistic missiles.
The ageing vessels, which have been in service for 30 years, are set to be replaced in the 2030s by the Dreadnought class, which are currently under construction.
In November, a Vanguard class submarine was reported to have had a near miss after a gauge malfunctioned and left it sinking towards an unsafe depth.
An MOD Spokesperson said: “HMS Vanguard and her crew have been proven fully capable of operating the UK’s Continuous At-Sea Deterrent, passing all tests during a recent demonstration and shakedown operation (DASO) – a routine test to confirm that the submarine can return to service following deep maintenance work.
“The test has reaffirmed the effectiveness of the UK’s nuclear deterrent, in which we have absolute confidence.
“During the test an anomaly occurred. As a matter of national security, we cannot provide further information on this, however we are confident that the anomaly was event specific, and therefore there are no implications for the reliability of the wider Trident missile systems and stockpile. The UK’s nuclear deterrent remains safe, secure and effective.”