[2] The class was introduced in 1994 as part of the Trident nuclear programme, and comprises four vessels: Vanguard, Victorious, Vigilant and Vengeance, built between 1986 and 1999 at Barrow-in-Furness by Vickers Shipbuilding and Engineering, now owned by BAE Systems.

In the early 1980s the British government began studies examining options for replacing the Resolution-class submarines and their Polaris missiles, both of which would be approaching the end of their service lives within little over a decade.

Options that were examined included:[7] A European solution and the US UGM-73 Poseidon were also briefly considered, but ultimately rejected, primarily on capability, cost and uncertainty grounds.

[7] Subsequently, on 10 July 1980, the then Prime Minister Margaret Thatcher wrote to US President Jimmy Carter requesting the purchase of Trident I missiles on a similar basis as the 1963 Polaris Sales Agreement.

However, following the acceleration of the US UGM-133 Trident II missiles, Thatcher wrote to US President Ronald Reagan in 1982 requesting the United Kingdom be allowed to procure the improved system instead.

The guidance drawings were then supplied for detailed design development by Vickers Shipbuilding and Engineering (VSEL) based at Barrow-in-Furness, now BAE Systems Maritime – Submarines.

This requirement led to the Vanguard-class design being significantly larger than the previous Polaris-equipped Resolution class, and at nearly 16,000 tonnes they are the largest submarines ever built for the Royal Navy.

[10] Beginning in 1985, both HMNB Clyde and the Royal Naval Armaments Depot Coulport at Faslane underwent extensive redevelopment in preparation for the Vanguard class submarines and Trident II missiles.

The work included enhanced "handling, storage, armament processing, berthing, docking, engineering, training and refitting facilities" at an estimates cost of £550 million.

On 4 December 2006, then Prime Minister Tony Blair revealed plans to spend up to £20 billion on a new generation of ballistic missile submarines to replace the Vanguard class.

[17] On 18 May 2011 the British government approved the initial assessment phase for the construction of a new class of four submarines, paving the way for the ordering of the first long-lead items and preparations for the main build to begin in the future.

[3] The reactor drives two GEC steam turbines linked to a single shaft pump jet propulsor, giving the submarines a maximum submerged speed of over 25 knots (46 km/h; 29 mph).

[25] On 16 March 2021 Prime Minister Boris Johnson unveiled his government's 10-year plan to boost international trade and deploy soft power around the world with an aspiration of creating a “Global Britain”.

In a document called Global Britain in a competitive age, this plan raised the cap on the number of nuclear warheads aboard the Royal Navy's Trident submarines from 180 to 260.