Mitsubishi Motors admitted Tuesday that it has been using an improper fuel-efficiency testing method for 25 years, widening a data-cheating scandal that has plunged the Japanese automaker into crisis.

"For the domestic market, we have been using that method since 1991," Mitsubishi vice president Ryugo Nakao told a Tokyo news briefing. "But we don't know the number of models" affected, he added.