The UK and Japan are to be able to deploy forces in each other's countries as part of a landmark new defence agreement.
Prime Minister Rishi Sunak signed a major defence treaty with Japanese Prime Minister Fumio Kishida at the Tower of London on Wednesday following years of negotiations.
Downing Street says the deal will cement the UK's commitment to Indo-Pacific security, allowing both forces to plan and deliver larger-scale, complex military exercises and deployments.
The UK is the first European country to have a Reciprocal Access Agreement (RAA) with Japan, the most important defence treaty between the UK and Japan since 1902.
The defence treaties will now be laid before Japan's Diet and the UK Parliament in the coming weeks.
The signing comes just weeks after the UK and Japan teamed up, alongside Italy, to develop the next generation of combat air fighter jets under the new Global Combat Air Programme.
Last month, the UK and Japan also launched a new UK-Japan digital partnership to strengthen co-operation across cyber resilience, online safety and semiconductors.
Mr Sunak said "in the past 12 months, we have written the next chapter of the relationship between the UK and Japan – accelerating, building and deepening our ties".
"We have so much in common: a shared outlook on the world, a shared understanding of the threats and challenges we face, and a shared ambition to use our place in the world for global good, ensuring our countries prosper for generations to come.
"In this increasingly competitive world, it is more important than ever that democratic societies continue to stand shoulder to shoulder as we navigate the unprecedented global challenges of our time."