Trident, also known as the Trident nuclear programme or Trident nuclear deterrent, covers the development, procurement and operation of submarine-based nuclear weapons in the United Kingdom. Its purpose, as stated by the Ministry of Defence, is to "deter the most extreme threats to our national security and way of life, which cannot be done by other means". Trident is an operational system of four s armed with Trident II D-5 ballistic missiles, able to deliver thermonuclear warheads from multiple independently targetable reentry vehicles. It is operated by the Royal Navy and based at Clyde Naval Base on the west coast of Scotland. At least one submarine is always on patrol to provide "continuous at-sea deterrence". The missiles are manufactured in the United States, while the warheads are produced by the British Atomic Weapons Establishment. Trident began patrols in 1994, and fully replaced its predecessor, the Polaris submarine fleet, in 1996.
The missiles have an intercontinental range of . Up to 16 missiles can be carried by each submarine, and each missile can carry eight warheads in MIRV configuration. The warheads have a yield of 100 kilotons of TNT, six times greater than the bomb that destroyed Hiroshima. The design, codenamed "Holbrook", is believed to be based on the American W76 warhead used on its own s.
The programme's acquisition cost was £12.52 billion (approximately £23 billion in 2024/25 prices), and its annual cost is estimated at £3 billion. The submarines' service life was designed to be 25 years but will exceed 36 years. In 2016, the House of Commons voted by a large majority to proceed with building a fleet of s, to be operational by 2028, and to phase out the Vanguard fleet by 2032. A new warhead, Astraea, is also planned to be introduced in the 2030s.
Critics argue that geopolitical threats do not necessitate the high cost of Trident, or that threatening to use strategic nuclear weapons is unethical and likely to violate international law. According to YouGov, , 50% of British adults support replacement of Trident with an equally powerful system.
The British government initially negotiated with the Carter administration for the purchase of the Trident I C-4 missile. In 1981, the Reagan administration announced its decision to upgrade its Trident to the new Trident II D-5 missile. This necessitated another round of negotiations and concessions. The UK Trident programme was announced in July 1980 and patrols began in December 1994. Trident replaced the submarine-based Polaris system, in operation from 1968 until 1996. Trident is the only nuclear weapon system operated by the UK since the decommissioning of tactical WE.177 free-fall bombs in 1998.
NATO's military posture was relaxed after the collapse of the Soviet Union in 1991. Trident warheads have never been aimed at specific targets on an operational patrol, but await co-ordinates that can be programmed into their computers and fired with several days' notice. Under the Nassau Agreement, British nuclear weapons are committed to the defence of NATO. From 1995 to some time before 2021, a 10 kiloton sub-strategic yield option was included on one missile per submarine.
Background During the early part of the Second World War, the United Kingdom (UK) had a nuclear weapons project, code-named Tube Alloys, which the 1943 Quebec Agreement merged with the American Manhattan Project to create a combined American, British, and Canadian project. The British government expected that the United States (US) would continue to share nuclear technology, which it regarded as a joint discovery, but the United States Atomic Energy Act of 1946 (McMahon Act) ended technical co-operation. Fearing a resurgence of US isolationism, and losing its own great power status, the British government resumed its own development effort. The first British atomic bomb was tested in Operation Hurricane on 3 October 1952. The subsequent British development of the hydrogen bomb, and a fortuitous international relations climate created by the Sputnik crisis, facilitated the amendment of the McMahon Act, and the 1958 US–UK Mutual Defence Agreement (MDA), which allowed Britain to acquire nuclear weapons systems from the US, thereby restoring the nuclear Special Relationship.
During the 1950s, Britain's nuclear deterrent was based around the V-bombers of the Royal Air Force (RAF), but developments in radar and surface-to-air missiles made it clear that bombers were becoming increasingly vulnerable, and would be unlikely to penetrate Soviet airspace by the mid-1970s. To address this problem, the UK embarked on the development of a Medium Range Ballistic Missile called Blue Streak, but concerns were raised about its own vulnerability, and the British government decided to cancel it and acquire the American Skybolt air-launched ballistic missile. In return, the Americans were given permission to base the US Navy's Polaris boats at Holy Loch in Scotland. In November 1962, the American government cancelled Skybolt. John F. Kennedy, then President of the United States, and Harold Macmillan, then UK Prime Minister, negotiated the Nassau Agreement, under which the US would sell Polaris systems for UK-built submarines, in exchange for the general commitment of the submarines to NATO. This was formalised in the Polaris Sales Agreement.
The first British Polaris ballistic missile submarine (SSBN), , was laid down by Vickers-Armstrongs at its yard at Barrow-in-Furness in Cumbria on 26 February 1964. She was launched on 15 September 1965, commissioned on 2 October 1967, and conducted a test firing at the American Eastern Range on 15 February 1968. She was followed by , which was completed by Vickers-Armstrongs on 29 September 1968; and two boats built by Cammell Laird in Birkenhead: , which was completed on 15 November 1968; and , which was completed on 4 December 1969. The four boats were based at HMNB Clyde at Faslane on the Firth of Clyde, not far from the US Navy's base at Holy Loch, which opened in August 1968. It was served by a weapons store at nearby RNAD Coulport. HM Dockyard, Rosyth, was designated as the refit yard for the 10th Submarine Squadron, as the Polaris boats became operational.
Polaris proved to be reliable, and its second-strike capability conferred greater strategic flexibility than any previous British nuclear weapons system. However it had a limited lifespan, and was expected to become obsolete by the 1990s. It was considered vital that an independent British deterrent could penetrate existing and future Soviet anti-ballistic missile (ABM) capabilities. An ABM system, the ABM-1 Galosh, defended Moscow, and NATO believed the USSR would continue to develop its effectiveness. The deterrent logic required the ability to threaten the destruction of the Soviet capital and other major cities. To ensure that a credible and independent nuclear deterrent was maintained, the UK developed an improved warhead package Chevaline, which replaced one of the three warheads in a Polaris missile with multiple decoys, chaff, and other defensive countermeasures. Chevaline was extremely expensive; it encountered many of the same issues that had affected the British nuclear weapons projects of the 1950s, and postponed, but did not avert, Polaris's obsolescence.
The Conservative Party had a strong pro-defence stance, and supported the British nuclear weapons programme, although not necessarily at the expense of conventional weapons. The rival Labour Party had initiated the acquisition of nuclear weapons, but in the late 1950s its left wing pushed for a policy of nuclear disarmament, resulting in an ambiguous stance. While in office from 1964 to 1970 and 1974 to 1979, it built and maintained Polaris, and modernised it through the secret Chevaline programme. In opposition in the early 1980s, Labour adopted a policy of unilateral nuclear disarmament.
More important than political differences was a shared sense of British national identity. Britain was seen as an important player in world affairs, its economic and military weaknesses offset by its membership of NATO and the Group of Seven, its then active membership of the European Union, its permanent seat on the UN Security Council, its leadership of the Commonwealth of Nations, and the nuclear Special Relationship with the US. To accept a position of inferiority to its ancient rival, France, was unthinkable. Moreover, the UK sees itself as a force for good in the world with a moral duty to intervene, with military force if need be, to defend both its interests and its values. By the 1980s, possession of nuclear weapons was considered a visible sign of Britain's enduring status as a great power in spite of the loss of the British Empire, and had become a component of the national self-image.
Negotiations
The Cabinet Secretary, Sir John Hunt briefed Cabinet on Polaris on 28 November 1977, noting that a possible successor might take up to 15 years to bring into service, depending on the nature of system chosen, and whether it was to be developed by the UK, or in collaboration with France or the US. With the recent experience of Chevaline in mind, the option of a purely British project was rejected. A study of the options was commissioned in February 1978 from a group chaired by the Deputy Under-Secretary of State at the Foreign Office, Sir Antony Duff, with the Chief Scientific Adviser to the Ministry of Defence, Sir Ronald Mason. The Duff-Mason Report was delivered to the Prime Minister, James Callaghan, in parts on 11 and 15 December. It recommended the purchase of the American Trident I C-4 missile then in service with the US Navy. The C-4 had multiple independently targetable reentry vehicle (MIRV) capability, which was needed to overcome the Soviet ABM defences.
Callaghan approached President Jimmy Carter in January 1979, who responded positively, but did not commit. The Carter administration's main priority was the SALT II Agreement with the Soviet Union, which limited nuclear weapons stockpiles. It was signed on 18 June 1979, but Carter faced an uphill battle to secure its ratification by the US Senate. MIRV technology had proved to be a major loophole in the 1972 SALT I Agreement, which limited numbers of missiles but not warheads. During the SALT II negotiations the US had resisted Soviet proposals to include the British and French nuclear forces in the agreement, but there were concerns that supplying MIRV technology to the UK would be seen by the Soviets as violating the spirit of the non-circumvention clause in SALT II.
Callaghan was succeeded by Margaret Thatcher following the general election on 3 May 1979, and she discussed the issue with Carter in October, who agreed to supply C-4, but he asked that the UK delay a formal request until December in order that he could get SALT II ratified beforehand. In the meantime, the MDA, without which the UK would not be able to access US nuclear weapons technology, was renewed for five more years on 5 December, and the MISC 7 cabinet committee formally approved the decision to purchase C-4 the following day. When Thatcher met with Carter again on 17 December, he still asked for more time, but the Soviet invasion of Afghanistan on 24 December ended all hope of Senate ratification of SALT II, clearing the way for the sale to proceed.
The British government hoped that Trident could be secured on the same terms as Polaris, but when its chief negotiator, Robert Wade-Gery, sat down with his American counterpart, David L. Aaron, in March 1980, he found this was not the case. Instead of the 5 per cent levy in recognition of US research and development (R&D) costs agreed to in the Polaris Sales Agreement, a 1976 law now required a pro rata fixed fee payment; the levy would have been approximately $100 million, however the fixed fee amounted to around $400 million.