The Royal Navy has taken delivery of XV Excalibur, the UK’s first uncrewed submarine, marking a major step forward in autonomous undersea warfare.
Officially classified as an extra-large uncrewed underwater vehicle (XLUUV), Excalibur is a 12-meter experimental vessel. At a displacement of 19 tons, it’s the largest uncrewed underwater vessel ever trialled by the Royal Navy.
The culmination of Project Cetus, it has been developed in under three years by the SDA in partnership with MSubs. Sponsored by the Royal Navy, Project Cetus aims to build ‘trust in autonomy’ and be a testbed to assess military payloads and missions.
In May 2025, Excalibur was named and unveiled at a ceremony in His Majesty’s Naval Base (HMNB) Devonport. It’ll now spend the next two years carrying out extensive sea trials, which will help understand the opportunities and challenges of operating an uncrewed vessel of this size.
Excalibur’s long-distance interoperability was demonstrated just months after the ceremony, in Exercise Talisman Sabre. During the exercise in August 2025, the Royal Navy successfully controlled Excalibur in UK waters from a remote operating centre in Australia, more than 10,000 miles from the vessel’s home in Plymouth.
Part of AUKUS Pillar 2, the “advanced capability” component of the AUKUS trilateral partnership, the exercise marked the first time the UK and Australia have demonstrated XLUUV interoperability as a single fighting force.
This was followed by a world-first trial, as Excalibur went to sea with a quantum optical atomic Tiqker clock on board developed by Infleqtion, a UK provider of quantum technology.
Related Article
World’s first: Quantum optical clock tested on underwater autonomous sub
Since launching in February 2025, Excalibur has completed many acceptance trials, surpassing several original design specifications.
Excalibur has now officially been handed over to the Royal Navy. The SDA’s Autonomy Unit will continue to support the Navy on Excalibur’s test and evaluation program, which aims to learn more about its capability and how we can effectively introduce autonomy to operational use.