The United Kingdom officially announced Tuesday that it will raise the cap on its nuclear arsenal to 260 submarine-launched ballistic-missile warheads from the previous target of 180 warheads.

“In 2010 the Government stated an intent to reduce our overall nuclear warhead stockpile ceiling from not more than 225 to not more than 180 by the mid-2020s,” according to a policy paper from Prime Minister Boris Johnson’s cabinet office, titled Global Britain in a Competitive Age. “However, in recognition of the evolving security environment, including the developing range of technological and doctrinal threats, this is no longer possible, and the UK will move to an overall nuclear weapon stockpile of no more than 260 warheads.”

The United Kingdom’s nuclear arsenal consists solely of U.S.-engineered warheads mounted to U.S.-made Trident ballistic missiles carried aboard British-built Vanguard-class nuclear-powered submarines. The U.K. eventually plans to replace its current Trident Holbrook warhead, a W76 variant, with a new warhead based on the planned U.S.-made W93. The Kingdom also aims to replace the Vanguard subs with Dreadnought class boats some time next decade.

Congress appropriated some $53 million in fiscal year 2021 for the National Nuclear Security Administration (NNSA) to conduct Phase 1 concept studies of W93. The semiautonomous Department of Energy nuclear weapons agency expects W93 to cost about $14 billion to build, in 2020 dollars, excluding the cost of the Navy-made Mk VII aeroshell that will deliver it. 

The W93 will be based on a nuclear-explosive design that was tested underground at full yield before the U.S. ceased such tests in the early 1990s, the NNSA has said. The next U.K. warhead will use the Mk VII aeroshell essentially off the shelf from the U.S., but not the exact same nuclear-explosive package as the U.S. W93.

During last year’s election-year budget deadlock in the U.S., the British government pushed hard to get the W93 funded. Democrats, who then controlled only the House but now control the entire U.S. government, voted to block funding for W93 in 2021.