THIS year marks the 50 year anniversary of the Continuous At Sea Deterrent, or CASD, the longest sustained military operation ever carried out by the United Kingdom.

Known as Operation Relentless, at least one Royal Navy ballistic submarine has been patrolling the world’s oceans, unseen and undetected since April 1969.

The commemoration, called CASD 50, was officially launched on January 16 this year at the naval base on the Clyde where Royal Navy submariners were presented with the new, silver, deterrent pin - awarded to those who have completed 10 or more patrols.

The history of the UK’s nuclear deterrent stretches back to 1958 and the signing of the UK and US Mutual Defence Agreement which saw the exchange of nuclear information, technology and materials.

It also led to an agreement on April 6 in 1963 for the UK to design and produce its own warheads and construct nuclear powered submarines to host the Polaris missile system.

Construction on the Resolution-class of ballistic submarines began soon after at Vickers Armstrong in Barrow and Cammell Laird in Birkenhead.

The first boat, HMS Resolution, was launched in September 1966 and completed a first deterrent patrol in June 1968.

By 1969 the UK began CASD with the four Resolution class boats, HMS Resolution, HMS Repulse, HMS Renown, and HMS Revenge.

They successfully completed a total of 229 patrols.

Work on the next generation of nuclear powered ballistic submarines also began with the first of class, HMS Vanguard, launched on 4 March 1992.

Sister boats HMS Vengeance, Vigilant and Victorious followed and the task of maintaining the deterrent was passed to this new generation.

All the Vanguard class boats are based at HM Naval Base Clyde and are operated by the Faslane Flotilla.

From 2020 the base will host all the Royal Navy’s attack submarines and will be the future home of the Dreadnought class of nuclear deterrent submarines, which will gradually replace the Vanguard class.