British nukes are utterly reliant on America

How long would it take for Britain to build its own?

“I USED TO joke”, says a former Pentagon official, “that the only thing British about British nuclear weapons is the fissile material. Everything else is American.” That is an exaggeration. But not by much. There are few areas where America and Britain are entangled as deeply as in the nuclear domain. How long would it take Britain to produce a home-grown bomb?

Britain’s deterrent comprises Trident D5 missiles carried on four Vanguard-class (and soon Dreadnought-class) submarines. Contrary to popular belief, they can be fired “completely independently” of America, says a naval officer familiar with the system, with no reliance on GPS for navigation. But for everything up to that point, Britain is dependent on its ally.

The bedrock of Anglo-American nuclear co-operation is the Mutual Defence Agreement (MDA) of 1958 and the Polaris Sales Agreement of 1963. These get around America’s draconian laws on sharing nuclear technology. No other country enjoys remotely similar arrangements. The MDA was signed only after the Russian Sputnik shock and when Britain had demonstrated its ability to build an H-bomb.

It is sometimes hard to “disentangle what is purely a US piece of information from a UK one”, says John Walker, a former British arms-control official. Take warheads. In theory, Britain designs its own at the Atomic Weapons Establishment (AWE) in Aldermaston. In practice, American and British warheads both have to fit into the same aeroshell, which shields them as they re-enter the atmosphere. Britain’s Holbrook warhead is derived from America’s W76; its next-generation Astraea A21 is closely linked to America’s new W93.

“To my best understanding the UK has never designed a modern thermonuclear weapon on its own,” says Jeffrey Lewis of the Centre for Nonproliferation Studies at the Middlebury Institute in Monterey. “The United States executes a design; that information is shared with the UK and they modify it.” A British official insists that the country could go it alone if needed. “We’ve got what we need out of them on Astraea,” he says. But even if AWE could produce an all-British design, it would still need America in other ways. “The non-fissile components are all basically produced in the United States,” says an insider.

Under the MDA, 955 non-nuclear parts of nuclear weapons were sent from America to Britain in 2020-23 alone, according to the Nuclear Information Service, a research group. That might have included so-called interstage materials, ultralight gels which transport radiation from the fission part of the bomb to the fusion part. “My understanding is those are only made in the United States,” says Mr Lewis, “and the United Kingdom does not have the capability to make them.” Britain also relies on America for tritium, a hydrogen isotope that boosts the explosive yield.

As for the missiles, Trident D5s are stored and refurbished in Kings Bay in the American state of Georgia; Britain leases and picks up its missiles from that common pool. “If we had to create a similar facility over here, that would be an extraordinarily expensive thing to do,” says a former official. Missile testing is a joint endeavour; when a British test failed spectacularly in 2024, it was because of a problem with the American test kit.

That is not to say co-operation is all one-way traffic. America’s nuclear labs have long welcomed technical input from Britain, says Mr Walker, with AWE serving as a “neutral” voice among the sprawling and sometimes cacophonous American ecosystem. “If there were differences between Livermore and Los Alamos,” he says, referring to two of the largest labs, “then getting a kind of third valid opinion from the UK was always welcome.”

If America were to sever all co-operation, Britain could hang on to one boatload of missiles at sea and another waiting to relieve it. That could buy a few years. What would a crash nuclear programme look like? The most ambitious and expensive effort would require building new ballistic missiles that could fit with Astraea, as well as production facilities for components and material and a new testing range. Reproducing “all the infrastructure” without America would cost more than £100bn, claims Dominic Cummings, a former adviser in Number 10.

A slightly simpler option would be to develop a nuclear-tipped cruise missile, a capability that Britain gave up in the 1990s. It would probably be launched from the air. A Trident warhead could not simply be strapped to cruise missiles—they are too big and would experience very different sorts of stresses in flight. But a new warhead would need to be designed and validated without explosive testing. Even if Britain were willing to walk away from the Comprehensive Test Ban Treaty, it would still have nowhere to test.

A government review in 2013 found that it would take 17 years to field a new Trident warhead and 24 years for a cruise-missile warhead. Those numbers were contested—a crash programme could surely be quicker. But the lack of testing is what would slow things down.

That leaves a third option: ask for help. In 1990 France offered to share the design of a nuclear warhead that would fit a French air-launched missile, notes James Jinks, a historian, in a new paper for Policy Exchange, a think-tank. Britain demurred, though it contemplated the French option as “one we might revert to later”. ■