The prime minister says the joint venture aims to create thousands of UK jobs and
strengthen security ties.
The nations will develop a next generation fighter - due to enter service in the
mid-2030s - that will eventually replace the Typhoon jet.
It is hoped the new Tempest jet will carry the latest weapons.
Work on developing it is already under way - with the aim to create a combat
aircraft that will provide speed stealth, use advanced sensors and even artificial
intelligence to assist the human pilot when they are overwhelmed, or under extreme
stress.
It could also be flown without a pilot's input if required and could be able to
fire hypersonic missiles.
But building such a complex aircraft is extremely expensive - developing the F35
jet was the most expensive programme ever undertaken by the Pentagon - so Britain
has been looking for partners.
Italy was already on board, and the addition of Japan is a significant move - at a
time when Britain is building closer ties with allies in the Indo-Pacific region
worried about a more assertive China.
Other countries could still join the programme. France, Germany and Spain are
already working together on their own separate design - as is the United States.