Why are three of the UK’s four nuclear deterrent submarines at sea?

The United Kingdom has simultaneously deployed three of its four nuclear deterrent submarines at sea for the first time in five years, The Times reported on Wednesday. At least one of the vessels – known as Vanguard-class submarines – was said to be armed with ballistic missiles which carry nuclear warheads. The report named the three submarines at sea as HMS Vanguard, HMS Vigilant and HMS Vengeance, with the fourth, HMS Victorious, currently undergoing maintenance. A navy source quoted by The Times described the deployment as “unusual”, without elaborating. The UK continuously deploys at least one of the four submarines as a security guarantee.