The UK and Japan have agreed a defence deal that will allow their national forces to "work more closely together", according to Prime Minister Boris Johnson.
The two nations have agreed a reciprocal access agreement (RAA) for armed services.
Number 10 said the "landmark" RAA will allow the two countries' forces to deploy together for training, joint exercises and disaster relief.
Downing Street said the agreement will boost the UK’s commitment to the Indo-Pacific and further safeguard global peace and security, and build on the already close collaboration on defence and security technology between both nations.
Speaking alongside his Japanese counterpart, Fumio Kishida, Mr Johnson said allies in Europe and east Asia need to be unified in the face of "autocratic, coercive powers".
The Prime Minister said the world has observed the "strong stance" the Japanese government has taken "against the Russian aggression in Ukraine".
He added: "We in the UK recognise that our security in Europe is indivisible from the security, our collective security, in the Asia-Pacific, in the Indo-Pacific region."
The statements came during Mr Kishida’s first official visit at the invitation of the Government.
The occasion was marked with a guard of honour and an RAF flypast over Horse Guards Parade in central London.
Before holding talks in No 10 on Thursday, the two leaders stood on a dais as they witnessed a Voyager and two Typhoon fighter jets soar over St James' Park and the parade ground.
Mr Kishida was then invited in Japanese by the captain of the Nijmegen Company, Grenadier Guards, to inspect the troops.
After the spectacle, Mr Johnson and Mr Kishida headed to Downing Street where the British leader announced that a military accord has been struck.
Talks began on deepening the UK-Japan defence relationship in September, following on from the Government’s defence and foreign policy integrated review which last year announced a "tilt" towards the Indo-Pacific.
The Government describes Japan as its closest security partner in Asia.