Suzuki Motor said it had used improper fuel economy tests for its cars in Japan, in a widening of a scandal that has engulfed Mitsubishi Motors.
But the company asserted that proper testing had shown the data did not need to be amended.
The news follows Mitsubishi's admission of fuel economy manipulation last month.
Japan's transport ministry had asked all Japanese car makers to re-submit fuel economy readings on all their vehicles by today.
Suzuki, Japan's fourth-largest car maker, said 2.1 million vehicles were affected but the company's chief executive Osamu Suzuki said that his workers did not intentionally use improper data.
"The company apologises for the fact that we did not follow rules set by the country," he said.
Shares in Suzuki ended over 9% lower after the company said it had used improper tests but before it briefed media. At one point they fell as much as 15% to their lowest level since November 2013.
Suzuki specialises in mini-vehicles, which have engines of up to 660cc and get preferential tax treatment under Japanese law. It commands roughly one-third of Japan's mini-vehicle market.
The car maker said it plans to continue sales of its cars given that new readings had not deviated much from those previously submitted, adding that it did not see much impact on earnings for now.
The impact of fuel tests has been much greater on Mitsubishi.
It has agreed to sell a one-third controlling stake to Nissan Motor in the wake of the scandal and said today that its president, Tetsuro Aikawa, will step down to take responsibility.
Emissions and fuel economy have come increasing scrutiny from regulators globally after Germany's Volkswagen admitted last year that it used "defeat devices" on 11 million diesel vehicles to meet emission rules.
France, which ordered tests on a random sample of about 100 diesel cars last year, said last month some vehicles made by Renault, Fiat, Mercedes, VW, Peugeot Citroen, Nissan, Opel and Ford failed to comply fully with its emissions regulations.
Meanwhile, Japan's Transport Ministry said that Mitsubishi Motors and Suzuki were the only car makers that used improper fuel economy tests for their cars in Japan.
A transport official also said the ministry had asked Suzuki to provide more information by May 31.
