Japanese car maker Mitsubishi has admitted that it manipulated fuel economy test data in more than 600,000 vehicles to make emissions levels look more favourable.

Tetsuro Aikawa, president of Japan's sixth-largest car maker by market value, bowed in apology at a news conference in Tokyo on Wednesday for what is the biggest scandal at Mitsubishi Motors since a defect cover-up more than a decade ago.

The company said the test manipulation involved 625,000 vehicles produced since mid-2013. These include its eK mini-wagon as well as 468,000 similar cars it made for Nissan Motor.

The problem was found after Nissan pointed out inconsistencies in data, the company said.

Mitsubishi shares plunged 15.16 percent to 733 yen ($6.73) after the announcement

Mitsubishi conducted an internal investigation and found that tyre pressure data was falsified to make mileage appear better than it actually was. 

Mitsubishi Motors is the first Japanese car maker to report misconduct involving fuel economy tests since Volkswagen was discovered last year to have cheated in diesel emissions tests in the United States and elsewhere.

South Korean carmakers § in 2014 agreed to pay $350m in penalties to the US government for overstating their vehicles' fuel economy ratings. They also resolved claims from car owners.

Source: Agencies