Boris Johnson is set to agree a "landmark" defence agreement with the Japanese prime minister in London which will allow for the two armed forces to work together.
Fumio Kishida's first official visit is to be marked with a guard of honour and an RAF flypast over central London today.
Downing Street said the leaders are expected to agree in principle a "landmark" reciprocal access agreement (RAA), paving the way for Japanese and British forces to work together on training, joint exercises and disaster relief activities.
It follows a "tilt" towards the Indo-Pacific announced in the Government's defence and foreign policy integrated review, published last year.
Mr Johnson said: "As two great island democracies, and the third and fifth largest economies in the world, the UK and Japan are focused on driving growth, creating highly skilled jobs and ensuring we remain technology superpowers.
"The visit of Prime Minister Kishida will accelerate our close defence relationship and build on our trade partnership to boost major infrastructure projects across the country - supporting our levelling up agenda."
Talks began on deepening the UK-Japan defence relationship in September last year.
Downing Street said the agreement would 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.
Last September, Australia, the UK and the US signed up to the Aukus pact to develop nuclear-powered submarines for Canberra in a bid to increase joint naval activity in the region.
The Government describes Japan as its closest security partner in Asia.
It says the island nation only has two other bilateral visiting forces agreements, one with the United States and the recently agreed RAA signed with Australia in January.
The two G7 leaders are also expected to discuss Russia's illegal invasion of Ukraine and how to exert pressure on President Vladimir Putin's regime.
Conservative former minister Greg Clark, MP for Tunbridge Wells, is also set to be announced as the UK's new trade envoy to Japan.
In addition, the two premiers are due to agree closer collaboration as part of the UK's clean green initiative.
Downing Street said it would allow the UK and Japan to work together to support countries in Asia to develop renewables and help provide alternative supplies to Russian oil and gas.
Japan is chair of the UK accession working group to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP).
The UK applied to join the free trade area in January last year.
During the meeting at Downing Street, the two leaders are expected to tuck into some food from the Fukushima region, including Japanese popcorn, which Downing Street said would be part of marking the lifting of remaining restrictions on food products from the Japanese region after the 2011 nuclear disaster.
