An internal investigation at Mitsubishi Motors has revealed that the company has failed to carry out proper fuel economy tests for more than 25 years.
The car manufacturer, which is thought to be the sixth largest in Japan, admitted that irregularities in its fuel testing procedure date back to 1991.
It comes after the firm found that fuel tests on some 625,000 sold in Japan had been manipulated to improve the appearance of efficiency rates.
Tetsuro Aikawa, president of Mitsubishi Motors, told reporters at a press conference that the exact scale of the issue is as yet unclear.
Speaking at Japan's transport ministry on Tuesday (April 26), he added that no decision can be taken on whether to compensate customers until it is known how many car are affected.
He said the investigation within the company is ongoing, which many have taken as a sign that more irregularities could still be uncovered.
Mr Aikawa said he does not know why employees falsified the test results, but added that he feels great responsibility.
The issue was revealed to affect some of the company's own-brand vehicles, as well as models it makes for Nissan.
It was discovered after Nissan found inconsistencies in the fuel economy figures and passed on its findings to Mitsubishi.
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