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The UK has, in the past, been lauded as one of the most transparent nuclear weapon states and the country that has taken the greatest strides towards disarmament since the end of the Cold War.

With a current stockpile estimated at 195 warheads, the UK is the smallest of the NPT-recognised nuclear weapon states (the P5) and the only country to have reduced to a single system: the submarine-launched Trident missile system.

The Government’s Integrated Review on Security, Defence, Development and Foreign Policy, published on 16 March 2021, announced some significant changes to the UK’s nuclear posture.

An increase in the UK’s nuclear stockpile

While the UK remains committed to a credible, minimum nuclear deterrent, the review concludes that the current security environment necessitates a move away from the previous decision in the 2010 SDSR to reduce the UK’s nuclear stockpile to no more than 180 warheads by the mid-2020s. The UK will now move toward an overall nuclear weapon stockpile of no more than 260 warheads, an increase of just over 40 per cent.

Following the change, the UK will remain the smallest of the NPT-recognised nuclear weapon states, but it will also join China as the only members of the P5 to be seen to be quantitatively increasing their nuclear stockpiles.

The reasons behind the Government’s announcement are the subject of some debate. A number of analysts have suggested that the increase is to accommodate the transition between the current Mk4/A warhead and the new replacement Mk7 warhead that was confirmed in February 2020. However, that replacement programme is only in its design phase and is not expected to enter service until the late 2030s. Any overlap between the decommissioning of the Mk4 and the entry into service of the replacement warhead is feasibly more than a decade away.

Others have suggested that the intention is to help persuade the US to move forward with its W93 warhead programme which is inextricably linked to the UK’s own programme.

There has also been some suggestion that the purpose of raising the stockpile cap is to allow for a more limited used of the deterrent, thereby utilising Trident’s “sub-strategic” role to address what the Integrated Review terms “the developing range of technological and doctrinal threats”.

A move away from transparency

To extend the UK’s position of deliberate ambiguity on the precise details of when, how and at what scale the UK may consider the use of its nuclear weapons capability, the review also confirms that the UK will no longer provide public figures on the UK’s operational stockpile, deployed warheads and deployed missile numbers.

The move has been met with some disappointment, with critics arguing that it decreases transparency, an important element of confidence building vis-à-vis other nuclear weapon states.

Security assurances

The Integrated Review says that security assurances extended to other countries by the UK are now also subject to review “if the future threat of weapons of mass destruction, such as chemical and biological capabilities, or emerging technologies that could have a comparable impact, makes it necessary”.

The inclusion of “emerging technologies” is new language in the Integrated Review that has prompted some commentators to suggest that nuclear weapons could be used in response to a cyber attack on the UK.

Implications for disarmament

The Government’s decision to increase the UK’s nuclear stockpile, and reverse decades of gradual disarmament progress, has met with anger and criticism from disarmament advocates who accuse the Government of contravening its disarmament obligations under Article VI of the Nuclear Non-Proliferation Treaty.

The UK will now have to justify its actions at the delayed NPT Review Conference, which is tentatively scheduled for August 2021.


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