TOKYO (REUTERS, BLOOMBERG) - Mitsubishi Motors Corp said its vehicles failed a fuel economy test, sending its shares down more than 15 per cent and wiping US$1.2 billion (S$1.54 billion) from its market value on Wednesday.

Mitsubishi Motors Corp. has admitted to manipulating test data in order to improve fuel-economy claims.

The manipulation involves 625,000 cars, including those supplied to Nissan Motor Co., according to a statement by the company. Nissan had discovered the issue and the two companies are discussing compensation.

Mitsubishi is Japan's sixth-largest automaker by market value.

"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 (S$9.02) 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.