Japan has warned that tens of thousands of British jobs with Japanese firms could be at risk if London pulls out of the European Union, a newspaper reported on Sunday.
Tokyo's submission to a British government consultation said Japanese companies liked Britain because it offered a gateway to the European market, the Sunday Times said.
Prime Minister David Cameron has vowed to renegotiate Britain's relationship with the EU and then hold a referendum on membership before the end of 2017 if he is still in office.
The Japanese government's submission to the British Foreign Office review said it was "committed to making its relationship with the EU stronger than ever before".
"In this context, it expects that the UK will maintain a strong voice and continue to play a major role in the EU," the Sunday Times quoted it as saying.
"The UK, as a champion of free trade, is a reliable partner for Japan. More than 1,300 Japanese companies have invested in the UK, as part of the single market of the EU, and have created 130,000 jobs, more than anywhere else in Europe.
"This fact demonstrates that the advantage of the UK as a gateway to the European market has attracted Japanese investment."
Japanese Prime Minister Shinzo Abe was in Northern Ireland last month for the G8 meeting hosted by Cameron.
Japan's embassy in London was quoted by the Sunday Times as saying that while some countries had not submitted comments, Tokyo thought it was appropriate because it is a major investor.
"If the UK leaves the single market, countries investing in the UK and exporting to the EU would have to pay tariffs, and that is not good news," the embassy was quoted as saying in a statement.
Close ally the United States has also previously warned Britain against isolating itself from the EU.