Britain and the United States have made military history by conducting their first live firing of a heavyweight torpedo from an uncrewed underwater vessel.
The weapon was launched from Excalibur, a 12-metre British-built undersea drone, during a trial in UK waters last week.
Downing Street said it was the first time either country had fired a torpedo from an uncrewed underwater platform rather than a crewed submarine.
The trial was conducted through AUKUS, the defence and security partnership bringing together Australia, the United Kingdom and the United States.
Heavyweight torpedo launched from Excalibur
The weapon used during the trial was a heavyweight torpedo jointly developed by the United States and Australia.
The Government described it as capable of targeting surface ships and submarines operating at significant depth.
Officials have not publicly identified the precise torpedo model, disclosed the firing location or released detailed results from the test.
The confirmed breakthrough concerns the launch platform: a full-size submarine weapon was successfully fired from an uncrewed underwater vessel without sailors aboard.
What is the Excalibur undersea drone?
Excalibur is an experimental extra-large uncrewed underwater vehicle developed in Plymouth for the Royal Navy.
The vessel is approximately 12 metres long and has been designed as a test platform for new sensors, payloads, autonomous systems and methods of underwater operation.
Unlike a conventional submarine, Excalibur does not carry a crew. It allows the Navy and its partners to test tasks which could eventually be performed without placing submariners directly in danger.
The platform is not an operational replacement for Britain's Astute-class attack submarines. It is a demonstrator intended to help defence planners understand how crewed and uncrewed systems could work together.
Trial conducted under AUKUS
The live firing formed part of AUKUS, the trilateral security partnership announced by Australia, Britain and the United States.
The programme is best known for plans to provide Australia with conventionally armed, nuclear-powered submarines, but it also includes cooperation on autonomous systems, artificial intelligence, undersea capabilities and other advanced defence technologies.
Using a US-Australian-developed weapon from a British-built underwater platform demonstrates the technical interoperability which the three countries are seeking through the partnership.
Downing Street said the successful trial represented a significant advance for future submarine technology across all three AUKUS nations.
Government claims NATO undersea advantage
The Government said the achievement would contribute to Euro-Atlantic security and strengthen NATO's advantage in undersea warfare.
Western governments have placed increasing emphasis on the underwater domain amid concern about hostile submarines and threats to infrastructure including communications cables, pipelines and energy connections.
Uncrewed systems could allow navies to maintain surveillance across wider areas, perform high-risk missions and deploy additional sensors or weapons alongside traditional vessels.
The technology also raises important questions concerning communications, cyber resilience, human control and the rules governing the use of armed autonomous or remotely operated platforms.
Not an autonomous strike announcement
The Government has confirmed a successful live launch but has not said that Excalibur independently selected or engaged a target.
It would therefore be inaccurate to describe the trial as proof that Britain has deployed a fully autonomous submarine capable of deciding for itself when to fire weapons.
No operational deployment date, procurement order or planned fleet size has been announced as part of the statement.
The test should instead be understood as a major technical demonstration showing that a heavyweight torpedo can be integrated with and launched from an uncrewed underwater platform.
Announcement made during Burnham's US visit
Prime Minister Andy Burnham announced the development while attending the United Nations General Assembly in New York.
The trial was revealed alongside a new UK-US partnership on defence artificial intelligence and autonomy, linking Britain's Rapid AI Delivery Taskforce with its American counterpart.
Although the initiatives share a focus on emerging defence technology, the AUKUS torpedo firing is a separate completed trial rather than merely a plan for future cooperation.
Burnham said British and American collaboration demonstrated what could be achieved when the two countries combined their expertise and innovation.
Future of uncrewed submarine warfare
The milestone indicates that extra-large underwater drones could eventually move beyond reconnaissance, mapping and surveillance to carry offensive weapons.
Future systems may accompany crewed submarines, operate in areas considered too dangerous for sailors or add capacity to a naval force without requiring the cost and personnel associated with a conventional submarine.
However, one successful firing does not establish that the technology is ready for routine operational use. Further trials will be required to test reliability, command arrangements, navigation, communications and safe weapon handling across realistic conditions.
The UK, US and Australia have not yet published a timetable for taking an Excalibur-type armed capability into service.