Prime Minister Shinzo Abe on Thursday warned that Britain would become "less attractive" for Japanese investment if it votes to leave the European Union in a membership referendum in June.
"A vote to leave would make the UK less attractive as a destination for Japanese investment," Abe said after talks with British counterpart David Cameron in Downing Street following visits to Germany and France.
"About 1,000 Japanese companies operate in the UK, employing 140,000 people... Many of the Japanese companies set up their operations in the UK precisely because the UK is a gateway to the EU," he said.
Cameron said Japanese investment in Britain totalled £38 billion (48 billion euros, $55 billion) at the end of 2014 and had played a particularly important role in reviving the British car manufacturing industry.
Nissan Sunderland is Britain`s biggest car plant.
"Japan is a country that matters enormously to the prosperity of the UK," Cameron said.
Abe said it was "better for the world" for Britain to stay in the European Union.
"Japan`s priority is negotiating trade deals with Europe, with the EU, a large trade area rather than individual states in Europe," he added.