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The UK Nuclear Warhead Stockpile - The Historical Question of Missiles

This essay was first published in October 2021, in the second volume of the Centre for Defence Studies series on The Integrated Review in Context: Defence and Security in Focus.

The March 2021 Integrated Review announced a change from the prior policy of reducing the UK’s overall nuclear warhead stockpile ceiling. Instead of ‘not more than 180 by the mid-2020s’ the new ceiling would be ‘no more than 260 warheads’. Instead of cutting back it would now be necessary to build up. The change was explained by reference to a ‘developing range of technological and doctrinal threats’. This was one of the most controversial aspects of the review. It implied a jump of some 45 percent in warhead numbers, a potentially significant leap, This was picked on by those who were already opposed to the nuclear force and also by those who worried that this sent the wrong signals, especially just before the next review conference of the 1970 Non-Proliferation Treaty (NPT).

More fundamentally it raised the question of what it means to have a ‘minimum’ independent deterrent.

It has been the policy of successive governments of both parties that the UK should remain a nuclear power but do so at the lowest level commensurate with a credible deterrent. – Professor Sir Lawrence Freedman

In the end, of course, credibility depends on the perceptions of the country being deterred and the circumstances in which deterrence is required. The deterrent is credible so long as the adversary (normally assumed to be Russia) accepts that there is a risk that it will be punished severely for any aggression.

Historical Context

Since the UK shifted to submarine-launched ballistic missiles (SLBMs) as the core of its nuclear strike force in the 1960s the minimum requirement considered essential for credibility is that one ballistic-missile-carrying submarine (SSBN) is on patrol at all times. Considerations of the reasonable length of patrols and the need for occasional long refits has led to the view that this requires four boats. The original intention was to have five boats but this was cut back to four by the Labour government in 1964. With each new generation of submarine (the Vanguard class in the 1980s and the Dreadnought class now) the question of whether three boats would be sufficient has been raised but as that would not leave much of a margin if one of the boats suffered major problems the level is now set at four.

Despite retaining the requirement for four SSBNs there has been a substantial reduction in the number of SLBMs carried. The Vanguard class SSBNs, which came into service in 1993, could carry up to 16 Trident D-5 submarine launched ballistic missiles, each of which could carry up to 8 warheads. That made for a maximum of 128 warheads. The initial intention was to purchase 65 American Trident II D5 missiles from the US, to be operated as a shared pool at the US Naval Submarine base at King’s Bay. The Labour Government’s 1998 Review announced that the order from the US would be cut back to 58 missiles. In 2006 after eight test firings the number was down to 50. It was decided not to purchase any more.

As the boats came into service in November 1993, Secretary of Defence Malcolm Rifkind announced that each boat would deploy no more than 96 warheads (ie 8 warheads per missile). The 1998 Strategic Defence Review halved this number - to 48 warheads. Then in 2010 the Coalition government cut it back further to 40. On this basis

there was little point in configuring the new Dreadnaught Class submarines, scheduled to enter service in the 2030s, for 16 missiles. The design has now been set at eight operational missile tubes.– Professor Sir Lawrence Freedman

During the 1970s there were over 400 nuclear warheads in the stockpile. The numbers went down slightly in the 1980s with the introduction of the Chevaline system for Polaris, designed to beat the Soviet anti-ballistic missile system around Moscow, including using dummy warheads. Then with the end of the Cold War the number of ‘operationally available warheads’ went down to 300. The 1998 Review put this down to 200. ThisRead the full collection here represented more than a 50% reduction in the number of weapons since the 1970s; the decline in explosive yield was even more substantial, to some 25% of that available in the 1970s. The objective was to reduce ‘the scale and readiness of our nuclear forces to ensure they are the minimum necessary to achieve our deterrent objectives.’ The 1998 Review stated that:

Although Trident is now our only nuclear weapon and covers both strategic and sub-strategic requirements, the potential explosive power deployed on a Trident submarine is one third less than a Polaris submarine armed with Chevaline.

This approach was taken even further in 2006 when the government decided to authorize work on the next generation of SSBNs. Now the number of available warheads in the stockpile was to be reduced ‘to fewer than 160’. There would also be a ‘corresponding 20% reduction in the size of our overall warhead stockpile’. Note the distinction between the total stockpile and the warheads available for operations. The operational total refers to warheads that are either onboard the available SSBNs or could be loaded quickly. The total stockpile included ‘a small margin to sustain the operationally available warheads’. It later transpired that the total overall stockpile was set at ‘no more than 225’. The Secretary of Defence at the time, Des Browne, reported that this would require dismantling some 40 warheads.

The 2010 review reduced the requirement for operationally available warheads from fewer than 160 to no more than 120 (sufficient for three SSBNs). The number for the overall nuclear weapon stockpile was reduced accordingly to ‘no more than 180’. The review reported that these changes would ‘start to take effect over the next few years. This will enable us to reduce our overall nuclear warhead stockpile ceiling from not more than 225 to not more than 180 by the mid 2020s.’ The next year Dr Liam Fox as Defence Secretary said that dismantling the 45 warheads had begun and would take around 15 years, implying taking out three warheads a year. A Guardian report in 2013 reported that warheads to be disassembled were stored at the RN Armaments Depot at Coulport on the Clyde or else were ‘work in progress’ at the Atomic Weapons Establishment at Burghfield. Some were modified so they could no longer be used while others no longer required for service were being stored without being disabled or modified. The 2015 review confirmed the process: ‘We will retain no more than 120 operationally available warheads and, by the mid-2020s, we will reduce the overall nuclear weapon stockpile to no more than 180 warheads, meeting the commitments set out in the 2010 SDSR.’

The Integrated Review and the Nuclear Deterrent

Thereafter progress does not appear to have been substantial. This helps explain the new position. The 2015 commitment would either have to be achieved during the period covered by the Integrated Review or else reappraised.

The simplest explanation for the target of 180 being abandoned is therefore that it has proved to be difficult or expensive to implement and this was the point at which this had to be acknowledged. – Professor Sir Lawrence Freedman

Because it was never achieved we can also note that the proposed increase in stockpile numbers is some 15 percent rather than 44 percent. This still does not explain, however, the need for the increase.

The simplest explanation is that with the new SSBNs due to enter service in the 2030s, and a new warhead under development, this is the appropriate moment to consider future requirement. The review justified the increase in the stockpile by reference to ‘the evolving security environment, including the developing range of technological and doctrinal threats’.

The government appears to require more operational warheads. As significant as the planned stockpile increase it is abandoning the commitment to no more than 40 warheads in an individual submarine, but without giving any new figure.– Professor Sir Lawrence Freedman

According to the Integrated Review:

Given the changing security and technological environment, we will …. no longer give public figures for our operational stockpile, deployed warhead or deployed missile numbers. This ambiguity complicates the calculations of potential aggressors, reduces the risk of deliberate nuclear use by those seeking a first-strike advantage, and contributes to strategic stability.

There are in practice limits on how many new warheads can be operationally deployed on a single SSBN. With the Dreadnaught Class boats being restricted to eight missiles tubes, then the potential for additional warheads on each boat is limited, although the American D-5 missiles can carry more than five warheads each. It may also be that the new SSBNs will be run more efficiently than the Vanguard Class and so two SSBNs can be on patrol more often.

Although this was not specifically mentioned in the review, in subsequent interviews Secretary of Defence Ben Wallace stressed the importance of improvements in Russian ballistic missile defences. He added:

‘In the past few years we have seen Russia invest strongly in ballistic missile defence . . .They have planned and deployed new capabilities and that means if we are going to remain credible, it has to do the job’.

When the UK responded to new Soviet missile defences in the late 1960s/early 1970s with the Chevaline programme the government had already decided not to go for the Poseidon SLBM which had multiple-independently-targeted warheads (MIRVs). As the D-5 is MIRVed extra warheads can beat the defences and there is no need to rely so much on decoys as used by Chevaline.

The second operational factor concerns the use of Trident in a sub-strategic mode. This is a somewhat misleading phrase as any nuclear use would be profoundly strategic. It is normally taken to mean against targets connected to an ongoing land battle. The Labour Government’s 1998 review argued that a sub-strategic capability was essential to the credibility of deterrence as ‘an option for a limited strike that would not automatically lead to a full-scale nuclear exchange.’ As WE 177 free-fall nuclear bombs intended for ‘sub-strategic’ use had been scrapped, Trident would now need to take on this role. A low kiloton warhead is available for that purpose.

The review noted that:

Some states are now significantly increasing and diversifying their nuclear arsenals. They are investing in novel nuclear technologies and developing new ‘warfighting’ nuclear systems which they are integrating into their military strategies and doctrines and into their political rhetoric to seek to coerce others.

The importance Russia gives to its short-range nuclear systems is a matter of debate. For the UK to use a Trident missile in response poses awkward operational issues, especially with regard to assuring that an SSBN will still have missiles available for ‘strategic’ use. – Professor Sir Lawrence Freedman

All operational scenarios can (thankfully) seem unlikely and fantastical but to those responsible they provide the basis for sizing the force, and endow the deterrent with some credibility.

Diplomatic and Political Implications

Against all these considerations there is the argument that the increase in numbers undermines the UK’s strong backing for the Non-Proliferation Treaty. The Defence Secretary insisted that nothing has changed. He told the Commons that the Attorney General had ruled ‘we do not believe that the changes to the number of warheads in any way breach the nuclear non-proliferation treaty’. There is a long-standing debate about the importance of Article VI of the NPT which looks to the declared nuclear powers ‘to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a Treaty on general and complete disarmament under strict and effective international control.’ The UK move will not help but there are bigger current issues to concern delegates to the next NPT Review conference, notably the end of the INF Treaty and the last-minute reprieve of New Start.

The last question is whether there are major political advantages to sticking to a minimum deterrent posture. The rationale for pushing the numbers as low as possible was made by Tony Blair in 2006:

We already have the smallest stockpile of nuclear warheads among the recognised nuclear weapons States, and are the only one to have reduced to a single deterrent system. In this White Paper we are announcing a further 20 per cent cut in our operationally available warheads. This leaves the deterrent fully functioning, with fewer than 160 warheads, but it means Britain continues to set an example for others to follow in our commitment to work towards a peaceful, fairer and safer world without nuclear weapons. Our decision to maintain the deterrent is fully compatible with all our international legal obligations.

The UK’s nuclear force would account for ‘less than 1% of the global inventory of nuclear weapons’. Its stockpile would be ‘the smallest of those owned by the five nuclear weapon States recognised under the Nuclear Non-Proliferation Treaty (NPT)’.

If this was an example none followed. France, unlike the UK, still maintains air-based systems and deploys more warheads on its SSBNs. Nor has the UK got much credit for its stance. For those opposed to nuclear weapons the only acceptable number for an arsenal is zero. Certainly the latest move has been condemned. David Cullen, the director of the Nuclear Information Service, described the move as ‘highly provocative’, adding ‘If they are tearing up decades of progress in reducing numbers, it will be a slap in the face to the 190 other members of the treaty, and will be regarded as a shocking breach of faith.’

Yet in 2006 when Blair made his case for a minimum deterrent, Cullen’s predecessor at the Nuclear Information Service, Di McDonald, observed that the reductions to date ‘have not been disarmament measures, they have been measures to remove old weapons that have become obsolete and they have been measures of efficiency’. It is instructive to look at other comments gathered by the Select Committee on Defence at the time: Paul Ingram, of BASIC: warhead reductions ‘almost irrelevant because we will still have 48 warheads out on patrol at any time’. Greenpeace: ‘the potential arsenal carried by a Vanguard submarine on patrol remains unchanged despite any wider stockpile changes proposed in the White Paper’. Bruce Kent, of CND: reductions in warhead numbers, though ‘certainly…welcome,’ more likely reflect ‘good housekeeping’. Dr Rebecca Johnson of the Acronym Institute for Disarmament Diplomacy: the new ceiling of 160 warheads ‘may…be little more than a political bid to make a virtue out of necessity’. The Committee stated that: ‘We welcome this arms reduction measure, but it is unclear whether this has significance as a non-proliferation measure. Since the White Paper proposes no changes to the number of warheads deployed on UK submarines, it is unclear that this reduction has any operational significance.’

As the government was given little credit for the past policy it might therefore have decided that there was little to lose in changing the policy. – Professor Sir Lawrence Freedman

There are important arguments to be had, both strategic and ethical, about the value of UK nuclear capabilities. So long as it is the government’s view that they are vital to the UK’s (and NATO’s) security then the logic of operational preparedness is hard to ignore. The numbers being discussed here, especially for weapons in storage, do not make an enormous difference either way. Until the late 1990s the size of the UK stockpile was a matter for speculation (and was normally exaggerated). The most relevant number is that of deployed missiles. They are now effectively capped by the limited number of missiles owned by the UK and the limited load of the next generation of SSBNs.


Professor Sir Lawrence Freedman is Emeritus Professor of War Studies, King’s College London. He was Professor of War Studies from 1982 to 2014 and Vice-Principal from 2003 to 2013. He was a member of the Iraq Inquiry, the official UK inquiry into Britain and the 2003 Iraq War, which reported in 2016.


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Lawrence Freedman

Lawrence Freedman

Emeritus Professor of War Studies

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