Since before 1997 the UK has operated a fleet of four ballistic missile submarines. One submarine is on patrol at all times, providing Continuous At Sea Deterrence (CASD). In 1997, the fleet was completing a transition from the Resolution class to the Vanguard class; this transition was complete when the fourth Vanguard class, HMS Vengeance, achieved full operational status in early 2001.
The WE177 free-fall nuclear bomb was withdrawn from service in 1998.
The 1998 Strategic Defence Review concluded that the UK needed a stockpile of fewer than 200 operationally available warheads, a reduction of a third from the maximum of 300 announced by the previous Government. The December 2006 White Paper: The Future of the United Kingdom’s Nuclear Deterrent (Cm 6994) announced a further reduction in our holdings of operationally available nuclear warheads, in line with the UK’s commitment to maintain only the minimum necessary deterrent; we now have fewer than 160 operationally available nuclear warheads.
On 24 September, the Prime Minister announced in his speech to the United Nations Security Council that, subject to technical analysis and progress in multilateral negotiations, when the next class of submarines enters service in the mid-2020s, the aim is for the UK to deliver CASD from a fleet reduced in size from four to three.