Britain’s Sovereign Warhead programme — a half and half hedging of bets
“We’ve got to have this thing over here whatever it costs…. We’ve got to have the bloody Union Jack flying on top of it” Ernest Bevin, UK Foreign Secretary, 1946.
Ernest Bevin’s words seem appropriate as we look at the UK strategic defence review released yesterday. His statement, made in 1946, when Britain had yet to develop the atom bomb but had seen the power of this new weapon and had seen it used against Nagasaki and Hiroshima, showed his recognition of the need for the United Kingdom to own and control this new weapon. Now, in 2025, we see the relevance of those words again.
President Trump’s obvious unwillingness to maintain the traditional United States “back stop” role in NATO, and his administration’s emphasis on Europe taking responsibility for its own defence as the US shifts its focus towards the Pacific, exposed Britain’s unique dependence on the United States nuclear deterrent. In the worst case scenario isolationist tendencies mean that Britain no longer benefits from having the United States as an ally that can be depended upon.
So, how did we get here?
In 1963, President Kennedy offered the Polaris missile solution to both Britain and France, and the British Prime Minister Harold Macmillan adopted it for the United Kingdom, agreeing the 1958 Mutual Defence Agreement. Fundamentally, at a time of straightened finances it was cheaper then developing a homegrown solution, and, strategically, it drew the British and the Americans closer together, which a post-war Britain needed. The French president, De Gaulle, went the other way, building an independent French atom bomb while making his disdain for the British decision clear, saying that the United Kingdom lived in a dependency culture with the USA.
Both the Polaris system, and its successor, the current Trident system, relies on a stealthy ‘Continuous At-Sea Deterrent’; the presence of a battery of nuclear missiles hidden under the ocean, providing a solid second-strike capability should Britain be attacked. Whilst the UK retains independence over the decision and capability to launch her Trident missiles, a hard dependency on American technical capability exists; the United States, via its contractor Lockheed Martin, both manufactures and maintains the actual missile systems operated by the UK.
The 1958 agreement has recently been extended to have no end-date. This looked like an attempt to shore up the so-called “special relationship” with the US in the face of Trump’s mercurial statements, but the uncomfortable truth remains that the dependency exists that, should the United States encounter problems with supply and manufacture of Trident, or “go slow”, or refuse to uphold its end of the agreement, then the UK would be left with a degrading and unmaintainable nuclear force.
Whilst couched in terms of Britain’s continuing commitment to NATO (the UK deterrent, unlike the French, is committed to NATO), the 2025 strategic defence review attempts to address this landscape.
The UK has announced that it will develop the a new nuclear warhead, designated the A21/Mk7, also known as “Astraea”. The Astrea warhead will be developed in parallel with the US development of their own revised warhead design (US W93/Mk7), with each nation creating its own design. We can assume the systems will not be too dissimilar, as they are both designed to work with the Trident missile system, launched from the Dreadnought class nuclear powered ballistic submarines that are being built to replace the current Vanguard class from the early 2030s.
The new warheads will be designed and tested (in conjunction with the French) using advanced computer simulation techniques, due to the moratorium on underground testing to which the UK remains committed. This means that the UK will have its own capability to design, build and maintain the “pointy end” of the nuclear stick.
But what of the stick itself?
The warheads, whether they are built by the United States or the UK will be launched via the Trident II D5 Strategic Weapon System, which is manufactured in the United States. Combined of a mixture of “on-submarine” systems and the missiles themselves, as well as on-shore support systems and training, this is not included in the sovereign warhead initiative. Presumably there are several factors going on here that include assumptions on being able to continue this relationship, the ability to replace or maintain the missile systems over time in the UK, the potential timescales for the missiles themselves to degrade in the event of supply problems and the desire to remain closely coupled with the US on defence matters.
The other element of interest in the recent defence review is the indication that the UK may seek to operate a mixed force of F-35A and F-35B models. This is significant, as the F35A is not currently in service in the UK and is the variant capable of delivering nuclear weaponry with a low or intermediate variable yield, specifically the B61 bomb. This signals a need address the issue that the current United Kingdom nuclear capability is an “all or nothing” option; the F35a may provide the ability to match aggression with like for like retaliation at different levels of the escalation ladder. This development is couched firmly in terms of supporting NATO capabilities and would not be supported by the development of an independent UK warhead; as such this cannot be launched without supply and support from the United States.
So, where does that leave the United Kingdom?
In some ways, nothing has changed. Britain remains dependent on the United States for its nuclear weapons deterrent for the foreseeable future and its capability is committed to NATO. The development of the sovereign warhead programme is a step towards a fully independent nuclear capability, should that be needed, but unless it is mirrored by decoupling the United Kingdom’s capabilities from the American missile delivery systems, it is symbolic only. It remains to be seen whether the UK uses this as the first stepping stone towards greater independence whilst not incurring the greater costs of replacing the entire weapons system, especially once delivery is underway, as in the case of the Dreadnought submarines. We can only assume that this may become the direction of travel should the NATO alliance change beyond all recognition or founder.
There is evidence here of the difficult choices that the United Kingdom government is struggling with. Since the Second World War, the United Kingdom has acted primarily with and supported by the United States. The advent of a two term Trump presidency, with all the uncertainty and disruption that that has caused, has undermined a lot of the underlying security assumptions made by successive governments since the 1940s. It is no longer sensible to assume that the United States will always be there to support Britain politically, technically and militarily; even when Trump inevitably disappears, whether through emergent isolationism or other priorities in the Pacific, the UK cannot bank on a Europe-focussed Washington.
The “half and half” approach of building a sovereign warhead looks like an attempt to strategically de-risk a part of the most technically difficult part of the nuclear deterrent while maintaining the cost effectiveness of the American systems. The rate at which highly technical maintenance is needed for the different elements may have been a factor in this calculation. Underlying the decision to bring only the warhead replacement “in house” at this time has to be the cost consideration; it is highly unlikely that the UK government could find the money to replace all elements of the end-to-end system in one programme. There is likely an element of “better to start somewhere than not at all”.
It is also likely that politics are in play here as well; there has been real criticism in recent months relating to the dependency on a potentially unreliable American ally. The “half and half” approach manages to remain within the status quo whilst also providing a solid example how such concerns might be mitigated that the government can point at.
In summary, the UK government is charting a tricky course, hedging its bets for a range of possible futures whilst accepting that the current reliance on the United States is unavoidable and, indeed, desirable in the near to medium term.
Should things go well, and NATO remains strong with US engagement, then and Britain can continue to benefit from the reduced cost of remaining a nuclear-armed power.
Most likely, should NATO remain viable but with a greatly reduced US military presence, then the addition of F-35a adds part of the capability that reduced US involvement might remove from Europe, allowing Russian tactical nuclear aggression to be met with a similar deterrent threat. The wider spectrum capability also carves out a value-add role for the UK as a part of NATO that genuinely adds to the overall European capability.
However, if the international situation develops in a more negative direction, with perhaps NATO becoming largely useless, in taking steps towards a more independent capability then Britain has at least given itself a chance of remaining a nuclear power in the medium to longer term by ensuring that nuclear expertise and experience remain available to future UK governments.
Bibliography
The Secret State, 2nd edition. 2010 Peter Hennessy
The Strategic Defence Review 2025 — Making Britain Safer: secure at home, strong abroad. 2025 UK Ministry of Defence.
https://assets.publishing.service.gov.uk/media/683d89f181deb72cce2680a5/The_Strategic_Defence_Review_2025_-_Making_Britain_Safer_-_secure_at_home__strong_abroad.pdf
Defence Nuclear Enterprise Command Paper: Delivering the UK’s Nuclear Deterrent as a National Endeavour. 2024 UK Ministry of Defence
https://assets.publishing.service.gov.uk/media/671b8641956d9b52e8c6d276/Defence_Nuclear_Enterprise_Command_Paper.pdf
The UK’s nuclear deterrent relies on US support — but there are no other easy alternatives
2025 Chatham House.
https://www.chathamhouse.org/2025/03/uks-nuclear-deterrent-relies-us-support-there-are-no-other-easy-alternatives
