British Navy Takes Delivery of XV Excalibur Autonomous Submarine for Atlantic Bastion Program.

The British Royal Navy has formally taken ownership of XV Excalibur, an extra-large uncrewed underwater vehicle, according to the UK Ministry of Defence and Submarine Delivery Agency on December 11, 2025. The move signals a shift from experimentation to operational testing as Britain builds out its Atlantic Bastion concept and expands autonomous undersea warfare.

The UK Ministry of Defence confirmed on December 11, 2025, that the Royal Navy has assumed ownership of XV Excalibur, an extra-large uncrewed underwater vehicle developed to explore how autonomous systems can operate alongside Britain’s nuclear-powered submarine force. The handover, managed through the Submarine Delivery Agency, transitions Excalibur from a development-focused program into an operationally oriented test platform intended to inform future doctrine, training, and procurement tied to the Royal Navy’s emerging Atlantic Bastion concept.
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On 15 May 2025, the Royal Navy unveiled its first Extra-Large Uncrewed Underwater Vessel at His Majesty’s Naval Base Devonport. (Picture source: UK MoD)

Officially classified as an Extra-Large Uncrewed Underwater Vehicle, Excalibur measures 12 metres in length and displaces 19 tonnes, making it the largest uncrewed underwater vessel ever trialled by the Royal Navy. Developed under Project Cetus, the platform has been delivered in under three years by the Submarine Delivery Agency in partnership with UK manufacturer MSubs Ltd, an unusually rapid timeline for a submarine-scale capability. Senior officials involved in the program described the pace as deliberate, reflecting urgent operational lessons drawn from undersea competition in the North Atlantic and beyond.

Excalibur’s naming and unveiling at His Majesty’s Naval Base Devonport in May 2025 signaled that the project had moved beyond laboratory experimentation. Since then, the vessel has completed a demanding sequence of acceptance trials following its launch in February 2025, with the Ministry of Defence confirming that several original design specifications were exceeded. Engineers familiar with the trials told Army Recognition that endurance, command-and-control responsiveness, and payload integration margins were among the areas that performed above baseline expectations, giving the Royal Navy greater flexibility for future mission profiles.

One of the most strategic milestones came during Exercise Talisman Sabre in August 2025. During the exercise, Royal Navy operators controlled Excalibur in UK waters from a remote operations center in Australia, more than 10,000 miles from the vessel’s home port in Plymouth. This demonstration of long-distance command and control marked the first time the United Kingdom and Australia had operated an XLUUV as an integrated capability, reinforcing AUKUS Pillar 2 ambitions to develop advanced, interoperable systems across the trilateral partnership. Defense officials involved in the exercise described it as a proof of concept for distributed undersea operations, where autonomous platforms can be task-managed across continents without exposing crewed submarines.

Excalibur has also become a testbed for technologies that directly support the Atlantic Bastion initiative, the Royal Navy’s evolving approach to securing the North Atlantic through layered undersea surveillance, deterrence, and sea denial. A notable example was a world-first sea trial involving a quantum optical atomic clock known as Tiqker, developed by UK quantum technology firm Infleqtion. Installed aboard Excalibur, the system was successfully operated underwater, representing the first deployment of such a device at sea within a submerged vessel. Unlike traditional microwave-based clocks, which can drift over time, the quantum clock provides vastly improved timing stability, allowing submarines and uncrewed platforms to navigate accurately without frequent reliance on GPS. For undersea forces operating in contested environments, this translates directly into longer endurance, greater stealth, and reduced vulnerability to detection.

The quantum trial, delivered six months ahead of schedule, underscored how Excalibur is being used not just as a vehicle but as a flexible laboratory for technologies that could later migrate onto crewed submarines or future autonomous fleets. Officials within the Submarine Delivery Agency’s Autonomy Unit emphasized that early integration and accelerated testing are essential if the UK is to maintain an edge in undersea warfare as peer competitors expand their own XLUUV programs.

With the formal handover complete, Excalibur will now enter an extended sea trial phase running through 2027. During this period, the Royal Navy, supported by the SDA’s Autonomy Unit, will evaluate how the platform can be employed alongside Astute-class and future Dreadnought-class submarines. Likely mission areas include intelligence, surveillance, and reconnaissance, seabed monitoring, payload delivery, and acting as a forward sensor node in contested waters. While Excalibur is not armed, its role as a pathfinder for autonomous tactics is seen as critical to shaping future force structure decisions.

For the UK defense establishment, Excalibur represents more than a single prototype. It is a tangible signal that Britain intends to remain a leading player in autonomous undersea technology, aligning industrial innovation, AUKUS cooperation, and operational doctrine. As one Royal Navy officer involved in the program noted privately, autonomy is no longer viewed as a distant supplement to submarines, but as an integral component of undersea dominance. The progress achieved with Excalibur in 2025 suggests that the Atlantic Bastion concept is moving from theory toward a credible, technology-backed reality.

Written By Erwan Halna du Fretay - Defense Analyst, Army Recognition Group

Erwan Halna du Fretay is a graduate of a Master’s degree in International Relations and has experience in the study of conflicts and global arms transfers. His research interests lie in security and strategic studies, particularly the dynamics of the defense industry, the evolution of military technologies, and the strategic transformation of armed forces.