UK Prime Minister Rishi Sunak and his Japanese counterpart Fumio Kishida will sign a "hugely significant" new defence agreement when the pair meet in London on Wednesday, Downing Street said.
The two leaders will also discuss Japan's current presidency of the G7 and "the need to maintain our collective support for Ukraine", according to a statement from Sunak's office.
The deal, to be signed at the historic Tower of London, will allow UK forces to be deployed to Japan in what London called "the most significant defence agreement between the two countries in more than a century".
"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," said Sunak.
"This Reciprocal Access Agreement is hugely significant for both our nations -- it cements our commitment to the Indo-Pacific and underlines our joint efforts to bolster economic security."
Kishida left on Monday for security focused talks with Japan's G7 allies in Europe and North America which ends with a meeting with US President Joe Biden on Friday.
In Paris, he and Emmanuel Macron pledged deeper ties, with the French president promising to maintain "joint actions in the Pacific" and France's "unfailing support" against North Korean aggression.
In turn, Kishida vowed G7 support for Ukraine.
The UK, Italy and Japan said last month that they will jointly develop a future fighter jet.
The new "Global Combat Air Programme" is slated to produce its first jets by 2035, merging the three nations' costly existing research into new aerial war technology, from stealth capacity to high-tech sensors.
A British patrol ship also last year took part for the first time in "Exercise Keen Sword", the regular Pacific training operation carried out by the Japanese and US navies.
The exercise involved 36,000 military personnel, 30 ships and 370 aircraft, drawn mainly from Japan and the United States but with Australia, Canada and the UK also joining in.
Japan has a pacifist post-war constitution, which limits its military capacity to ostensibly defensive measures.
But Tokyo is poised to make the largest overhaul to its security strategy in decades, in the light of the war in Ukraine, repeated missile launches from North Korea and growing pressure from China.
The leaders are also expected to discuss trade, including the UK's possible accession to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP).