Mitsubishi Motors Corp said its vehicles failed a fuel economy test, sending its shares down more than 15 per cent and wiping $1.2 billion from its market value on Wednesday.

Japan's sixth-largest automaker by market value said its president, Tetsuro Aikawa, will attend a news conference at 5:00 pm (0800 GMT) to address issues of misconduct related to those tests.

"Our vehicles were found to have failed part of a fuel economy test," a spokesman for the automaker said. Mitsubishi Motors declined to comment further. No one at Japan's transportation ministry was immediately available to comment.

Shares in Mitsubishi Motors closed down more than 15 per cent at 733 yen each, the stock's biggest one-day drop in almost 12 years.

Citing unnamed sources, national broadcaster NHK said the issue affected around 600,000 mini-vehicles produced under the Mitsubishi brand, mainly in Japan, including the eK model, as well as a model it produces for Nissan Motor.

This is the first time a Japanese automaker has reported misconduct involving fuel economy tests since Volkswagen last year was discovered to have cheated diesel emissions tests in the United States and elsewhere.

South Korean car makers Hyundai Motor Co and affiliate Kia Motors Corp in 2014 agreed to pay $350 million in penalties to the US government for overstating their vehicles' fuel economy ratings. They also resolved claims from car owners. - Reuters