The fighting warplanes will be interoperable so that the allies will be able to fight together in future conflicts if needed.
Japan, Britain and Italy are set to jointly develop a next-generation fighter jet in a project that holds scope for future cooperation with allies.
To be ready by 2035, the new jet is expected to merge the nations' current research into cutting-edge air combat technology, from stealth capacity to high-tech sensors.
In a joint statement, the three countries said the "ambitious endeavour" would "accelerate our advanced military capability and technological advantage" at a time when "threats and aggression are increasing" worldwide.
Their announcement was accompanied by a set of images showing an artist's impression of the sleek new jets flying past Mount Fuji and over London and Rome.
They did not give a cost estimate, but the three countries are already pouring billions of dollars into high-tech fighter jet development, efforts that will come together under the joint project called the Global Combat Air Programme.
"We share (an) ambition for this aircraft to be the centrepiece of a wider combat air system that will function across multiple domains," the statement said.
That includes "future interoperability with the United States, with NATO and with our partners" in Europe, Asia and worldwide, it explained.
On December 9, the leaders of Japan, the UK, and Italy announced the joint development of a next-generation fighter aircraft. This cooperation will contribute to global peace and stability. (* The images for illustration purposes only.) https://t.co/SlgnTcqjGb pic.twitter.com/mKEcaPIDii— Japan Ministry of Defense/Self-Defense Forces (@ModJapan_en) December 9, 2022
In a separate joint statement with Japan's defence ministry, the US Department of Defense said it supported the project.
"We have begun important collaboration through a series of discussions on autonomous systems capabilities, which could complement Japan's next fighter program among other platforms," the US-Japan statement said.
Better than F-35
The announcement comes with Japan poised to make the most significant overhaul to its security strategy in decades.
The government plans to ramp up defence spending - a controversial move in a nation whose constitution limits military capacity to ostensibly self-protective measures.
But the war in Ukraine, repeated missile launches from North Korea and growing pressure from China have helped build support for a bigger budget.
A Japanese defence ministry official said the new fighter jet aimed to exceed the capabilities of existing models like the United States' F-35.
While the total cost has not been finalised, it will not be borne precisely equally between the three countries, the official added.
Japan's Nikkei business daily said that three companies -- Mitsubishi Heavy Industries, BAE Systems and Leonardo -- would oversee the new project, Tokyo's second joint development after its SM-3 missile made with Washington.
The project is the latest high-profile example of allied countries collaborating on an ad-hoc basis to develop defence equipment.
However, such moves proved controversial last year when the United States snatched a lucrative contract to supply Australia with submarines from under French noses and launched a new US-UK-Australia alliance in the Pacific, dubbed AU KUS.