Japanese automaker Mitsubishi on Wednesday admitted it manipulated pollution test data in more than 600,000 vehicles, after reports of misconduct sent its Tokyo-listed shares crashing earlier in the day.

The company said it would halt production and sales of the affected vehicle models -- mini-cars sold in Japan -- and launch a probe into cars sold overseas.

The embarrassing admission comes in the wake of a massive pollution-cheating scandal at Volkswagen that erupted in September and which the German giant is still struggling to overcome.

"We found that with respect to the fuel consumption testing data... (the company) conducted testing improperly to present better fuel consumption rates than the actual rates," Mitsubishi president Tetsuro Aikawa told a Tokyo news briefing.

He added that "the testing method was also different from the one required by Japanese law".

"We express deep apologies to all of our customers and stakeholders for this issue," Aikawa said.

The problem affected about 625,000 vehicles, including mini-cars "eK Wagon" and "eK Space", and the "Dayz" and "Dayz Roox", which Mitsubishi produces for rival Nissan.

The rigged data was discovered after Nissan found inconsistencies in fuel-economy data and reported the apparent improprieties.

Mini-cars are small vehicles with 660cc gasoline engines that are hugely popular in the Japanese market, but have found little success abroad.

Mitsubishi, which makes the Outlander sport utility vehicle, sold more than a million vehicles at home and overseas in its latest fiscal year.

"Taking into account the seriousness of these issues, we will also conduct an investigation into products manufactured for overseas markets," the carmaker said in a statement.

"In order to conduct an investigation into these issues objectively and thoroughly, we plan to set up a committee consisting of only external experts. We will publish the results of our investigation as soon as it is complete."

Earlier Wednesday, Mitsubishi shares plunged 15.16 percent to 733 yen ($6.73) after local media reported the faulty emissions tests.

The fall was its biggest one-day plunge since 2004, Bloomberg News reported.

At that time, Mitsubishi was struggling to launch a turnaround as it teetered on the edge of bankruptcy, hit by a lack of cash and a series of huge recalls.

German titan Volkswagen has been hammered by a global scandal that erupted when it emerged that it had installed emissions-cheating software in 11 million diesel engines worldwide.

The costs of the scandal are still incalculable but are expected to run into several billions of dollars as a result of fines and lawsuits.

South Korean automakers Hyundai and Kia in 2014 agreed to pay $100 million to settle a US government investigation into exaggerated fuel efficiency on 2012 and 2013 car models sold in the United States.