The UK Ministry of Defence’s Submarine Delivery Agency (SDA) officially handed over the extra-large uncrewed underwater vehicle (XLUUV) Excalibur to the Royal Navy in December 2025. Described by the MoD as an “autonomous submarine,” Excalibur measures 12 metres in length and weighs approximately 19 tonnes, making it the largest uncrewed underwater vehicle ever trialled by the Royal Navy. The platform was formally named and unveiled during a ceremony held at His Majesty’s Naval Base Devonport in Plymouth in May 2025.

Excalibur demonstrated its strategic potential during Exercise Talisman Sabre in August 2025. During the exercise, Royal Navy operators successfully controlled the vehicle in UK waters from a remote operating centre in Australia—more than 16,000 kilometres from its home base in Plymouth. This achievement marked the first time that the UK and Australia demonstrated XLUUV interoperability as a single fighting force. According to the MoD, this milestone represents significant progress under Pillar 2 of the AUKUS defence partnership, which focuses on advanced capabilities such as autonomy, artificial intelligence, and undersea warfare technologies.

The handover of Excalibur represents the culmination of Project Cetus, which has been under development since 2022. The programme has been led by the SDA in close partnership with UK-based manufacturer MSubs Ltd. Project Cetus was designed to build confidence in naval autonomy while providing a flexible testbed for evaluating military payloads, sensors, and mission concepts relevant to future uncrewed undersea operations.

According to a statement released by the Ministry of Defence on 11 December, Excalibur has already completed a series of acceptance trials, during which it surpassed several of its original design specifications. Following its delivery, the platform will now enter an extensive sea trial and evaluation phase lasting until 2027. Throughout this period, the SDA’s Autonomy Unit will continue to support the Royal Navy as Excalibur is used to refine operational concepts and inform the development of next-generation uncrewed underwater systems.