Tens of thousands of British jobs with Japanese firms could be at risk if the UK pulled out of the European Union, the government in Tokyo has indicated.
Its submission to the Foreign Office's review of the relationship with Brussels said Japanese firms were attracted to the UK because it offered a gateway to the prized European market.
The Sunday Times reported that the memo from the Japanese government expects the UK to maintain a "strong voice" in Brussels.
David Cameron has committed to renegotiate the UK's relationship with Brussels and then hold an in/out referendum on staying in the 28-member bloc before the end of 2017 if the Tories win the general election.
It added: "In this context, it expects that the UK will maintain a strong voice and continue to play a major role in the EU.
"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."
In a statement to the Sunday Times the Japanese embassy in London said: "We know some countries decided not to submit comments but as a non-EU nation and major investor in the UK we thought it was appropriate.
"We have taken advantage of this occasion to express our expectations ... 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."
But eurosceptic Tory MP Julian Brazier told the newspaper: "It's kind of the Japanese to give us guidance on our national destiny but at a time when the eurozone is in a crisis unseen for nearly a century I'm not sure they are well qualified to see the way forward for Britain. In deciding on our national interest we may have to disappoint their expectations."