Prime Minister Rishi Sunak Thursday announced Japanese investment of more than $22 billion into the UK, but accepted carmakers' concern about Brexit trade rules as he confirmed talks with the European Union.

Visiting Tokyo ahead of a G7 summit in Hiroshima, Sunak said the investment by Japanese companies of £17.7 billion ($22.1 billion) was a "massive vote of confidence" in the UK economy.

It includes planned funding of about £10 billion for offshore wind and hydrogen energy projects from Japanese trading house Marubeni, Downing Street said.

Sunak said the deals showed the fruits of post-Brexit Britain becoming the first non-regional economy to accede to the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP).

But the UK's divorce from the EU remains a serious headache for companies.

European carmaker Stellantis has warned it could close some UK factories should Sunak's government fail to tweak Brexit trade rules, to ease the supply of batteries for electric vehicles.

Ford, Jaguar Land Rover and German car industry lobby group VDA have all reportedly joined in calls for tariff-free access to be maintained despite a looming deadline under the EU-UK Brexit deal.

"It's something that car manufacturers across Europe, not just in the UK, have raised as a concern," Sunak told British media in Japan.

"And as a result of that we are engaged in a dialogue with the EU about how we might address those concerns when it comes to auto manufacturing more generally," he said.