Sir Keir Starmer said the UK and Japan were building a “new era of co-operation” as the two countries agreed a multi-billion-pound investment deal.
The prime minister hosted his Japanese counterpart Sanae Takaichi for talks on Sunday ahead of the G7 summit.
“UK and Japan unlock significant inward investments totalling more than £9 billion in infrastructure and financial services and up to £9 billion in offshore wind,” Downing Street said in a statement.
The £18bn deal is expected to create tens of thousands of new jobs, giving a boost to the British economy.
Earlier, the Department for Science, Innovation and Technology said that a “landmark tech partnership” between the UK and Japan had been agreed and backed by businesses in both countries.
It said that a formal partnership had been agreed “for the first time” connecting “the UK’s world-class microchip design expertise with Japan’s advanced manufacturing”.
The collaboration between the UK Semiconductor Centre and Rapidus, Japan’s 2nm semiconductor manufacturing facility, would create a “direct pathway for the UK semiconductor sector to manufacture cutting-edge chips”, it added.
Rapidus is backed by £11.6bn in Japanese government investment and the department said the agreement marked a “critical step in strengthening the resilience of the UK semiconductor sector”.
Sir Keir said the UK-Japan Frontier Technology Partnership was “combining UK excellence in R&D and software with Japanese advanced manufacturing experience and expertise”.
He also said the UK and Japan were “working more closely” on defence and had “lots of issues to discuss in a volatile world” ahead of the G7 summit next week.
He hailed the UK-Japan Defence Capability and Industrial Council as “hugely important” and said the Gcap fighter jet development initiative was at the “heart of the relationship between our two countries”.
“I’m delighted that we’re building stronger cooperation, a new era of cooperation between our two countries,” he said.
This came after Sir Keir’s leadership survival plans were rocked by the shock resignation of John Healey as defence secretary over a dispute about long-term funding for the military.
Mr Healey was joined in his exit from government by armed forces minister Al Carns as well as two parliamentary aides, heaping pressure on the prime minister, who was already facing the threat of a leadership challenge.
Ms Takaichi said the UK and Japan were “strengthening” security cooperation and had a “shared strategic recognition”.
“Today, Japan and the UK hold a firm determination to contribute together to peace and stability in the international community, and are highly important, like-minded countries,” she said.
“I believe that our relationship has reached a level that could even be described as quasi-allies.”
A Downing Street spokesperson said the prime minister also welcomed “significant new Japanese investment into the UK, supporting tens of thousands of jobs, including in real estate and financial services”.
“The leaders discussed the conflict in the Middle East, Russia’s illegal invasion of Ukraine, stability in the Indo-Pacific region and the importance of resilient global supply chains and free trade,” the spokesperson added.
Technology minister Kanishka Narayan said Japan “is one of our closest tech partners”.
“This deal will offer our talented innovators driving the future of quantum computers, next-generation chips, and clean energy direct access to vital investment, manufacturing and markets,” he added while discussing the UK-Japan Frontier Technology Partnership.
“At the same time the UK’s world-class research base, software expertise, and pro-innovation environment makes us the ideal partner for Japanese firms looking to expand and invest. “By working together we can bolster growth and jobs across our countries and build a stronger, more secure economy that improves lives across the UK.”