The deal will make the UK the first European country to have such an agreement with Japan. It is part of the UK’s post-Brexit defence and foreign policy “tilt” towards the Indo-Pacific region.
The pact comes amid rising tensions with China, including its threats to invade Taiwan.
No. 10 called it the most important defence treaty between London and Tokyo since 1902.
Rishi Sunak and Japanese prime minister Fumio Kishida signed the deal at the Tower of London before holding talks.
The Prime Minister, speaking to Mr Kishida at the signing, referenced UK accession to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership and the war in Ukraine as he welcomed the Japanese leader to the Tower of London.
“It is fantastic to welcome you here to London and to the United Kingdom, it is a great pleasure to have you here at the Tower of London.
“The relationship between our two countries is stronger than ever, not just across trade and security but also our values and I think we saw that brilliantly demonstrated last year,” he said.
Mr Sunak offered “wholehearted support” for Japan as it prepares to host the G7 later this year, while also telling his counterpart that the UK “warmly welcomes the ambition in your national security strategy”.
The formal signing ceremony took place at the centuries old Thames-side castle, where Mr Kishida was greeted by a guard of honour of Yeomen Warders, also known as Beefeaters.
Governor of the Tower of London, Brigadier Andrew Jackson, and the Constable of the Tower of London, Sir Gordon Messenger, were both present to welcome the two leaders.
Mr Kishida said: “Let us have a strategic discussion, that would be our hope,” as he referred to the importance of security and co-operation between the two countries.
During the visit to the castle, Mr Sunak and Mr Kishida were shown Japanese armour presented to King James VI in 1613 by the then-Shogun Tokugawa Hidetada of Japan to mark the first trade agreement between England and Japan.
Under the terms of the defence agreement British troops sent to Japan could face the death penalty if convicted of capital crimes, after British negotiators failed to get immunity from such a punishment for visiting personnel.
Following their meeting a Downing Street spokeswoman said:
“Collaboration across defence and security would not only benefit Japan and the United Kingdom, but broader global stability, the leaders agreed.
“Discussing Japan’s leadership of the G7 in such unprecedented times, the Prime Minister welcomed Prime Minister Kishida’s plans to focus on the impact of the invasion of Ukraine on global food and economic security.
“Reflecting on the UK’s accession to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP), a free trade bloc with a combined GDP of £9 trillion, the Prime
Minister said it offered the UK a unique opportunity to join a group of like-minded countries who shared similar values.
“The Prime Minister added that future growth and prosperity for all countries would be driven by collaboration and innovation, and both leaders agreed there was huge potential for growth across the two countries’ economies.
“The Prime Minister looked forward to visiting Hiroshima for the G7 in May.”