Atomic Pulse

Reversing Course: The UK’s Decision to Increase the Cap on Nuclear Warheads

Earlier this week, the UK government shocked nuclear observers by announcing it will increase the cap on its overall nuclear weapon stockpile from “not more than 180 by the mid 2020s” to “no more than 260 warheads” – a 44 percent increase from the previous target. This decision represents a significant shift from the previous domestic cross-party consensus on reducing the number of nuclear weapons. In another surprise move, the UK government also announced it will no longer publish details of its nuclear stockpile and missile numbers.

Coming just months before the Nuclear Non-Proliferation Treaty (NPT) Review Conference in August 2021, these announcements will have major negative implications for global perceptions of the UK’s commitment to the NPT, nuclear reductions, and transparency. They also present a significant complication for the Biden Administration’s nuclear policy agenda, which aims to reduce the role of nuclear weapons in national security strategy and work with allies to move toward a declaratory policy stating that the sole purpose of nuclear weapons is to deter (or retaliate against) nuclear attacks.


The UK has steadily reduced its nuclear arsenal from Cold War peaks of approximately 500 nuclear warheads. Today, according to the UK House of Commons Library, the UK’s maintains a stockpile of 195 warheads on one system – nuclear-armed ballistic missiles on submarines – of which a maximum of 120 warheads are considered “operationally available.” The UK is the only recognized nuclear weapon state that currently operates with a single deterrent system. The system operates under “continuous at-sea deterrence,” meaning at least one submarine with nuclear weapons aboard is always deployed.

Since the signing of the 1958 U.S.-UK Mutual Defense Agreement (MDA), the UK’s deterrent has been heavily reliant upon the United States. The Trident missiles that would be used to launch the warheads are U.S.-made and kept in a common pool at the King’s Bay Naval Submarine Base in Georgia. While the warheads themselves are designed and developed independently at the UK’s Atomic Weapons Establishment (AWE), it is commonly understood that the warhead is at least partially based on U.S. designs.

The UK is currently embarking on a nuclear modernization program to replace its four Vanguard class SSBNs (the new submarines are known as the Dreadnought Class), delivery systems, and accompanying warheads. The W93, a new proposed U.S. warhead, is set to be used to guide the development of Britain’s new warhead.

Increasing Warhead Cap

In 2010, the David Cameron government pledged to reduce the UK’s overall nuclear weapon stockpile to no more than 180 warheads by the mid-2020s. This was reiterated in a 2015 security review. These policies appeared to have cross-party support in the UK Parliament (though some political parties, such as the Scottish National Party and a portion of the Labour Party, believe the UK should not modernize its deterrent and instead should move more quickly toward nuclear disarmament).

The latest review cites an “evolving security environment, including the developing range of technological and doctrinal threats” as the reason for increasing that cap to 260 warheads. The specific citing of “technological and doctrinal threats” is likely a reference to Russia’s new nuclear systems and its alleged “escalate-to-deescalate” policy. No explanation is given for how increasing the total number of UK warheads will alleviate the stated concerns or improve the security of the UK.

Bottom Line

The UK government’s decision to increase the cap on nuclear warheads represents a marked and abrupt shift from previous commitments and raises many questions both domestically and for the international community. As the NPT Review Conference approaches and the Biden Administration seeks to reduce the role of nuclear weapons, the UK likely will face increasing criticism from many sides. Nonetheless, more time will be needed to fully understand and assess the consequences of this decision.

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