The US and UK test-fired a heavyweight torpedo from an unmanned submarine for the first time.
The US and Australia jointly developed the torpedo, and the UK launched it as part of the AUKUS agreement.
Western countries are rushing to modernise their war-fighting capabilities in the face of threats from Russia and Iran.
Britain and the United States have for the first time test-fired a torpedo from an uncrewed undersea vessel, according to the British government.
It said in a statement on Monday, local time, the launch took place in UK waters last week as part of the AUKUS alliance with Australia.
It said the test marked "a major advance in the future of submarine technology" for the three countries and delivered "a significant breakthrough for Euro-Atlantic security and NATO's undersea advantage".
Facing threats from Russia and Iran, Western countries are rushing to modernise their war-fighting capabilities with autonomous systems.
The heavyweight torpedo — jointly developed by the US and Australia — was launched from a 12-metre-long undersea drone manufactured in Britain and named XV Excalibur.
The UK and Australia carried out a joint test-drive of Excalibur in 2025 as part of Exercise Talisman Sabre.
An operating centre in Australia, more than 16,000 kilometres from Excalibur's home in Plymouth, took remote control of the vessel, in what the British Royal Navy described as "a big step forward in our ability to operate uncrewed systems interchangeably with Australia".
Sidharth Kaushal, a senior research fellow at the RUSI think-tank, called the recent torpedo test-fire "noteworthy".
"The employment of a lethal payload illustrates that uncrewed systems can play roles which extend beyond surveillance," he told news agency AFP.
The announcement came as UK Prime Minister Andy Burnham headed to New York for the United Nations General Assembly.
Mr Burnham is due to meet US President Donald Trump at the summit, the first time since he became the British leader in July.
The UK government said Mr Burnham would use his trip to New York to "drive forward global work" on artificial intelligence.
He will announce "a new AI and autonomy partnership" with the US that will "help the UK protect critical national infrastructure, from sea lanes to airspace", a statement added.
"This week I will be making the case for closer international cooperation on AI, a just and lasting peace in Ukraine, and renewed efforts to bring an end to the conflict in the Middle East because what happens abroad impacts each and every one of us at home," Mr Burnham said.
He also confirmed Britain would host the G20 summit in November 2027, in the north-western English city of Manchester.