There are fears Nissan and Hitachi, who operate in the North East, could move operations out of the UK if Brexit makes it a less profitable place to do business.
Prime Minister Theresa May has addressed the G20 summit in China, saying she is confident the UK can build and maintain its business links with countries like Japan after leaving the European Union.
More than 6000 people are employed at the Nissan plant in Sunderland, with up to 30,000 more in the supply chain. Nissan says its position on the matter remains unchanged and it continues to monitor the situation.
Japanese ambassador Koji Tsuruoka says Japanese companies will leave the UK if they no longer feel it is profitable.
But he says they are trying to ensure that the companies will continue to operate because many of the Japanese industry leaders like working here.