Japanese automaker Mitsubishi Motors shares have fallen more than 15% after it said an unspecified number of its cars had failed fuel tests.
Mitsubishi will hold a news conference in Tokyo at 5pm local time about the issue.
Mitsubishi shares closed down 131 yen at 733 yen in Tokyo – their biggest one-day fall in nearly 12 years.
NHK said the faulty tests could affect about 600,000 Mitsubishi-produced cars, including some vehicles it makes for rival Nissan.
Mitsubishi sold more than one million vehicles in 2015.
“One of our models was found to have failed part of a fuel economy test,” a company spokesman said.
In 2014 South Korean auto makers Hyundai and its affiliate, Kia, agreed to pay $350 million in US penalties for overstating their vehicles’ fuel economy ratings. They also resolved claims from car owners.
The Mitsubishi revelation follows last year’s emissions scandal at Volkswagen in which the German automaker was found to have installed devices in some models that fooled tests.