Project CETUS: The Royal Navy's Giant Autonomous Submarine

As global navies race to develop autonomous systems, the Royal Navy is quietly advancing one of the UK's most ambitious defence technology programmes.

Known as Project CETUS, the initiative is exploring how large uncrewed underwater vehicles (XLUUVs) could transform naval operations over the coming decades.

Rather than replacing conventional submarines, Project CETUS is designed to demonstrate what autonomous underwater platforms can achieve alongside crewed fleets. From intelligence gathering to seabed surveillance, the programme offers a glimpse into the future of maritime defence.

What is Project CETUS?

Project CETUS is a technology demonstrator commissioned by the UK Ministry of Defence to develop an Extra Large Uncrewed Underwater Vehicle (XLUUV) for the Royal Navy. The vehicle has been built by Plymouth-based MSubs Ltd under the oversight of the Submarine Delivery Agency.

Photo: Royal Navy / inside Cetus

Measuring approximately 12 metres in length and weighing around 19 tonnes, CETUS is one of the largest autonomous underwater vehicles developed in Europe. Unlike traditional submarines, it carries no crew, relying instead on advanced autonomous navigation and mission management systems.

Its primary purpose is experimentation rather than operational deployment. The Royal Navy intends to use the platform to evaluate how autonomous underwater systems can support future maritime operations in increasingly contested environments.

Why Does It Matter?

Modern naval warfare is evolving rapidly. Undersea infrastructure—including communications cables, energy pipelines and offshore installations—has become strategically significant. At the same time, advances in autonomous technology are creating new opportunities to conduct missions without placing personnel at risk.

Project CETUS aims to address these challenges by investigating how autonomous submarines could:

- Conduct long-duration intelligence, surveillance and reconnaissance (ISR) missions

- Support anti-submarine warfare (ASW)

- Monitor and inspect critical underwater infrastructure

- Deploy specialist sensors or mission-specific payloads

- Operate in high-risk environments where sending a crewed submarine may be undesirable

Because the platform is modular, different payloads can be integrated depending on mission requirements, making it a flexible testbed for future capabilities.

Built for Experimentation

Unlike an operational submarine, CETUS is not intended to carry weapons as part of its current demonstration programme. Instead, it provides engineers and military planners with a platform to test autonomous technologies, navigation systems, endurance, communications and operational concepts.

The demonstrator entered sea trials in 2025 and was later formally named XV Excalibur, marking another milestone in the Royal Navy's autonomous systems programme.

The results of these trials will help determine how future generations of XLUUVs could be integrated into Britain's underwater force.

Supporting the Royal Navy

One of the biggest advantages of autonomous underwater vehicles is persistence.

Crewed submarines are among the most capable assets in any navy, but they are expensive to build, complex to operate and limited by crew endurance. Autonomous systems can undertake routine or hazardous missions while allowing conventional submarines to focus on tasks that require human judgement.

Potential future applications include:

- Persistent surveillance over strategic maritime areas

- Inspection of seabed infrastructure

- Environmental and hydrographic surveys

- Mine countermeasure support

- Force protection around naval bases and offshore installations

Rather than replacing Astute-class or future SSN-AUKUS submarines, XLUUVs are likely to complement them by extending the Royal Navy's operational reach.

Part of a Wider Modernisation Effort

Project CETUS forms part of the Royal Navy's broader investment in autonomous systems and digital technologies.

Alongside programmes such as PROTEUS and SCYLLA, CETUS contributes to the UK's long-term vision of integrating autonomous platforms across the maritime domain. These initiatives also support wider collaboration with allies through partnerships such as AUKUS, where advanced technologies—including artificial intelligence and autonomous systems—are becoming increasingly important.

Challenges Ahead

Despite its promise, autonomous underwater technology still faces significant technical hurdles.

Reliable underwater communications remain difficult, autonomous decision-making must be robust and secure, and cyber resilience is becoming a critical consideration. Long-duration power management, navigation without GPS and legal frameworks governing autonomous military systems are also areas that require continued development.

Project CETUS provides an opportunity to address many of these questions before larger fleets of autonomous underwater vehicles become operational.

Looking to the Future

The Royal Navy has described CETUS as an experimentation platform rather than an end product. However, the knowledge gained from the programme could influence underwater operations for decades to come.

As autonomous technologies continue to mature, large uncrewed underwater vehicles may become a routine part of naval task groups—conducting surveillance, protecting critical infrastructure and supporting crewed submarines on complex missions.

Project CETUS represents an important step towards that future. While much of its work will remain behind the scenes, its success could help define how the Royal Navy operates beneath the waves in the years ahead.

Author's Note: Project CETUS demonstrates a broader shift in defence thinking: using autonomous systems to enhance capability rather than replace personnel. As the technology evolves, programmes like CETUS will provide valuable insights into how humans and autonomous platforms can operate together in one of the world's most challenging environments.

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