The UK's nuclear deterrent relies on US support - but there are no other easy alternatives

27th March 2025

Now that Washington is potentially an unreliable ally, the UK needs to revisit its nuclear strategy. But going alone is costly, and partnering with France poses its own risks.

The Trump administration's volatile approach to its allies has forced the UK to reconsider fundamental pillars of its defence policy, including whether it can rely on the US. This extends even to the UK's nuclear deterrent, with many questioning whether the UK needs a nuclear weapons system that is less dependent on the US for maintenance and support.

Unlike most US allies in NATO, the UK is officially a nuclear weapons state, and therefore less reliant on Washington's nuclear extended deterrence guarantee. It has its own nuclear weapons system, Trident, which is based in Britain and ostensibly operates independently.

However, Trident is closely linked to the US's nuclear programme, raising concerns about its independence. The missiles are US-built, and the system relies on the US for maintenance.

Trident is also expensive, absorbing about 6 per cent of the UK's defence budget in 2023 - though changes to the way UK Defence classifies spending on nuclear issues means it's now difficult to say how much Trident on its own costs every year. Aside from the cost, alternative nuclear deterrent strategies - including partnership with France or a wider European scheme - pose their own challenges.

The UK government therefore faces a difficult choice, with no easy options.

How Trident relies on US support
Trident is comprised of the UK's four Vanguard class nuclear-powered, nuclear-armed submarines. The UK deploys one of these submarines at all times, in line with its ‘Continuous At-Sea Deterrent' policy, or CASD.

The submarines are based at Faslane, near Glasgow, and are currently being replaced by new Dreadnought submarines, being built at Barrow-in-Furness in Cumbria.

In day-to-day operations, the UK’s Trident programme is completely independent from the US. The British Prime Minister can launch the missiles without any external input.

Trident is closely linked to the US’s nuclear programme, raising concerns about its independence. The missiles are US-built, and the system relies on the US for maintenance.

However, while operationally independent, Trident is a US-built missile system and the UK’s nuclear capabilities currently rely on the US in several ways. The UK invested in Trident’s research and development, and purchased the missiles from the US under the terms of the modified Polaris sales agreement, updated in 1982 to also cover Trident. The Trident missiles rely on the US for maintenance which is done by the manufacturer Lockheed Martin; missiles have to return to the US for scheduled maintenance every few years. The UK also purchases the aeroshells required for producing nuclear warheads from the US.

The US-UK mutual defence agreement which underpins this cooperation - including the missile maintenance - was extended indefinitely in 2024 and includes clauses that make it difficult to terminate the arrangement. However, there is still the risk that the US could decide to ignore the agreement or threaten to do so as a form of leverage.

The UK’s options - go solo, France, or Europe?
The UK has three realistic potential options to reduce reliance on the US for its nuclear deterrent.

The first would be to build up a domestic industrial capability for producing submarine-launched ballistic missiles (SLBM) to replace Trident. This would be a lengthy and expensive endeavour. The current arrangement with the US is more cost effective, as the US has the advantage of economies of scale in design, production and maintenance that would not apply to a domestic UK SLBM programme.

Another option would be technical collaboration with France, the other nuclear power in NATO aside from the US. France uses a similar type of strategic nuclear-powered, nuclear-armed submarine to the UK, and is likewise in the process of developing a replacement for its current system.

This option may face issues as the two systems have different technical foundations. But procuring French missiles could be a quicker option than developing domestic capabilities, if they could be made to work in the British submarines.

However, this approach would still involve reliance on an ally. While France and the UK are aligned on European security questions for now, cooperation is not guaranteed. For example, Marine Le Pen strongly opposes France sharing its nuclear deterrent and would likely reject weapons sharing if she came to power.

A third option could be to spread the costs of nuclear systems among European allies. This could take place within NATO, with the UK and France receiving financial compensation for providing nuclear capabilities to the organisation - especially if in the absence of US capabilities. While this would not address the UK’s core reliance on the US, it could help build a short-term arrangement within Europe and provide funds that the UK could use to develop an alternative system.

To date, France has been reluctant to accept financial support out of concerns it could imply restrictions on its nuclear sovereignty. However, if France, the UK and other European allies could come to an agreement, it would have the advantage of spreading costs while maintaining a nuclear capability within NATO.

Alternative deterrents
This dilemma also taps into wider questions about the effectiveness of nuclear weapons systems.

Given it is widely understood that the UK would only use nuclear weapons in extreme scenarios, their effectiveness as a deterrent is ambiguous. Nuclear weapons might deter a nuclear strike, but their role in deterring conventional aggression is less clear.

Authors
Dr Marion Messmer
Senior Research Fellow, International Security Programme
Olivia O’Sullivan Director, UK in the World Programme

Source - https://www.chathamhouse.org/2025/03/uks-nuclear-deterrent-relies-us-support-there-are-no-other-easy-alternatives

https://www.chathamhouse.org/