Japanese automaker Mitsubishi Motors has admitted falsifying fuel economy data for more than 600,000 vehicles sold in Japan.

The company has admitted that tire pressure figures were falsified by employees to flatter mileage rates.

Almost 470,000 vehicles that Mitsubishi made for Nissan were affected and the issue was uncovered after Nissan found inconsistencies.

The announcement sent shares in Mitsubishi down more than 15% in Tokyo.

Mitsubishi bosses, including the company’s president Tetsuro Aikawa, bowed deeply at the start of a press conference on April 20 in Tokyo.

“The wrongdoing was intentional. It is clear the falsification was done to make the mileage look better. But why they would resort to fraud to do this is still unclear,” Tetsuro Aikawa said.

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Although he was unaware of the irregularities, the company’s president said: “I feel responsible.”

The inaccurate tests involved 157,000 of Mitsubishi’s own cars and 468,000 vehicles produced for Nissan.

The issue affected models including Mitsubishi’s ek Wagon and eK Space, as well as Nissan’s Dayz and Dayz Roox.

All are “mini-cars” with 660cc petrol engines and are popular in Japan but have found little success in other markets.

Mitsubishi Motors is Japan’s sixth-largest car maker and sold more than one million vehicles in 2015.

The issue was reported to Japan’s transportation ministry and Nissan told dealers to stop selling the affected vehicles. It was considering ways to help owners of the affected cars.

Shares in Mitsubishi Motors closed down 131 yen at 733 yen in Tokyo – their biggest one-day fall in nearly 12 years.

This is the first time that a Japanese automaker has reported misconduct involving fuel economy tests.