UK's new Astraea nuclear warhead to equip future Dreadnought submarines in the 2030s.

The British Ministry of Defence officially confirmed that funding was secured under a £15 billion nuclear investment plan for the Project Astraea, which aims to develop the United Kingdom’s next-generation nuclear warhead.

On October 20, 2025, British Defence Minister Luke Pollard confirmed that Project Astraea, the United Kingdom’s next-generation nuclear warhead programme, is fully funded within a £15 billion government investment plan. The Astraea A21 warhead will replace the Mk4A Holbrook currently deployed on Trident II D5 missiles and is intended to arm the Royal Navy’s Dreadnought-class submarines entering service in the 2030s. Developed by the Atomic Weapons Establishment, the programme is included in the Strategic Defence Review 2025 as part of the government’s effort to maintain a continuous at-sea deterrent under Operation Relentless.
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The Astraea will replace the Holbrook nuclear warhead currently deployed on Trident II D5 missiles carried by Vanguard-class submarines, and later by their Dreadnought-class successors. (Picture source: British MoD)

Responding to a parliamentary question from Ben Obese-Jecty, the British Ministry of Defence officially confirmed that Project Astraea, the United Kingdom’s next-generation sovereign nuclear warhead, is advancing as part of the Strategic Defence Review 2025. Defence Minister Luke Pollard stated that the Astraea programme, also designated as A21/Mk7, is fully funded within the government’s £15 billion investment into the sovereign nuclear warhead initiative during this Parliament. This total also covers the sustainment of the current Mk4A Holbrook warhead and the modernisation of infrastructure at the Atomic Weapons Establishment (AWE). Pollard added that the projected cost of Astraea itself will remain undisclosed for national security reasons. The A21/Mk7 warhead is being designed, developed, and manufactured in the United Kingdom by AWE, which was nationalised in 2021.

Project Astraea is being developed in coordination with the United States’ W93 warhead programme to ensure continued compatibility with the Trident II D5 system used by both nations. This cooperation includes shared use of certain non-nuclear components, such as the Mark 7 aeroshell, under long-standing treaty frameworks established through the 1958 Mutual Defence Agreement and the 1963 Polaris Sales Agreement. The Mark 7 aeroshell defines specific parameters for mass distribution and dimensions of the internal warhead, allowing interoperability between U.S. and U.K. systems while preserving sovereign control of the nuclear explosive package itself. The Strategic Defence Review 2025 states that Astraea’s development forms part of the United Kingdom’s “National Endeavour” across the Defence Nuclear Enterprise, supporting approximately 9,000 jobs nationwide. The programme’s purpose is to maintain a credible, minimum, and independent nuclear deterrent that is continuously deployed under Operation Relentless through the Royal Navy’s Continuous At-Sea Deterrent.

The design of Astraea incorporates technological advances developed through the U.K.–U.S. Joint Technology Demonstrator, particularly in safety, surety, and materials resilience. It will feature insensitive high explosives and other safety measures to ensure reliability under a range of operational conditions. Astraea will be the first British nuclear warhead developed and deployed without live nuclear testing, in accordance with the United Kingdom’s obligations under the Comprehensive Nuclear-Test-Ban Treaty. Validation of its performance relies on simulation and advanced experimentation using the AWE’s Orion laser, which operates with 10 long-pulse and two petawatt short-pulse beams to recreate conditions up to ten million degrees Celsius. Computational analysis is performed using the Valiant supercomputer, while hydrodynamic and radiographic testing takes place at the Franco-British EPURE facility in Valduc, France, as part of Project Teutates established under the Lancaster House Treaties. This combination of facilities allows the AWE to certify warhead designs without nuclear testing, ensuring compliance with current treaties while maintaining credibility of the deterrent.

To sustain long-term production capacity and correct previous cost overruns, AWE has reorganised its industrial infrastructure. The Future Materials Campus (FMC), planned for procurement in 2025, merges several major projects previously conducted separately, including the Pegasus, Aurora, and Mensa facilities. The Pegasus project, originally paused in 2018 and resumed in 2021, is intended for enriched uranium processing and is now expected to cost approximately £1.7 billion, with completion projected by 2030. The Aurora facility for plutonium components remains in the early design phase with an estimated cost of £2.3 billion. The Mensa warhead assembly and disassembly facility at Burghfield has been delayed seven years, increasing from its original £734 million budget to around £2.16 billion. By consolidating these efforts, the Future Materials Campus is intended to stabilise production schedules, strengthen quality control, and address workforce shortages across the defence nuclear sector.

According to information released in the Strategic Defence Review, Astraea’s yield is likely to fall between that of the American W76-1 warhead, at approximately 90 kilotons, and the W88, at around 475 kilotons. This represents a potential increase in yield compared to the United Kingdom’s current 100-kiloton Holbrook warhead. The design will incorporate resistance to radiation and electromagnetic interference, including hardening against cold X-rays, and may feature built-in countermeasures against advanced missile defence systems. The Mark 7 aeroshell and other shared components will ensure proper weight balance and flight stability for Trident II D5 integration. The system is also expected to use new arming, fuzing, and firing mechanisms designed for improved reliability and safety margins, as well as updated security protocols to prevent unauthorised activation or tampering. These design choices aim to ensure Astraea’s operational compatibility with both existing and future iterations of the Trident missile system while maintaining independence of the U.K. nuclear deterrent.

The Strategic Defence Review 2025 reaffirms that a modernised nuclear deterrent remains central to the United Kingdom’s defence strategy and its commitment to NATO. It notes that Russia’s expanding nuclear capabilities and reliance on coercive deterrence, as well as China’s rapid nuclear build-up, are key factors requiring sustained investment in the Defence Nuclear Enterprise. The review also outlines the policy of deliberate ambiguity on the operational stockpile, deployed warhead, and missile numbers to preserve deterrent uncertainty. The United Kingdom’s nuclear weapons are operationally independent but assigned to NATO defence, with launch authority reserved solely to the Prime Minister. The document also calls for strengthened cooperation with France in technical, political, and policy areas, building upon the Lancaster House Treaties, while maintaining the long-standing partnership with the United States through shared research, materials exchange, and coordination of deterrence postures within the Euro-Atlantic framework.

The Strategic Defence Review outlines that the Defence Nuclear Enterprise must ensure stable delivery of the deterrent through a long-term financial and industrial approach. It specifies that the Ministry of Defence will maintain ringfenced funding for the nuclear portfolio to protect it from other departmental pressures, guaranteeing continuity across programmes including Astraea and the Dreadnought-class submarine fleet. Astraea’s development is described as a multigenerational effort requiring consistent leadership, skills retention, and stable industrial partnerships across the defence supply chain. The government’s goal is to bring Astraea into service in the 2030s alongside the first Dreadnought submarine, ensuring that at least one ballistic missile submarine remains continuously deployed at sea at all times. The combination of the A21/Mk7 warhead and the Dreadnought-class platform is expected to secure the United Kingdom’s nuclear deterrent capability well into the second half of the century, forming the cornerstone of the nation’s strategic defence posture.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.