Japan has given a stark warning to Britain that its big companies -- and others -- would have to quit the country if trade barriers after Brexit wipe out their profit margins.

After meeting Prime Minister Theresa May with a delegation of top Japanese executives, Japan's ambassador to the U.K., Koji Tsuruoka, was asked about the threat of new obstacles to trade between Britain and the European Union, the country's biggest export market.

"If there is no profitability of continuing operations in the U.K. - not Japanese only - then no private company can continue operations," he told reporters.

"So it is as simple as that," the official added. "This is all high stakes that all of us, I think, need to keep in mind."

Japan is a big investor in Britain. There are more than 1,000 Japanese companies doing business in the country, employing more than 140,000 people.

They include global banking giant Nomura (NMR) and top automakers Toyota (TM), Nissan (NSANF) and Honda (HMC).

Japan went public with a detailed list of concerns about the impact on its companies shortly after Brits voted to leave the EU in a referendum in June 2016.

Related: Britain's most painful Brexit choice is all about trade

Japanese firms want reassurances that the U.K. will seek to limit any "harmful effects" on their businesses, according to a document published by the government in Tokyo.

Their wishlist includes maintaining the current zero tariffs on goods moving between the U.K. and the rest of Europe, as well as the existing customs procedures and the "passport" system that allows financial services firms based in London to do business across the EU.

European officials have made clear that Britain's plan to leave the EU internal market and customs union will inevitably mean new barriers in some of those areas.

The top European executive from Nissan, which employs nearly 7,000 people at the U.K.'s largest car factory, attended Thursday's meeting with May, along with representatives from Hitachi (HTCMY), Toyota, Honda, Mitsubishi, SoftBank (SFTBF) and Mizuho (MFG).

"Business representatives ... agreed on the importance of the time-limited [Brexit] implementation period in providing clarity and certainty for people and businesses," a spokesperson for May's office said.

"There was also agreement on the importance of moving quickly in the negotiations to secure a trading relationship with the EU that is as tariff-free and frictionless as possible following the implementation period."

-- Charles Riley contributed to this article.