Embattled automaker Mitsubishi Motors Corp. admitted Tuesday that its method of testing fuel efficiency, which does not comply with Japanese standards, has been in use since 1991, much longer than it had previously stated.

The Tokyo-based automaker said it is investigating why its testing specialists have used the unapproved method for a quarter-century.

"We are not sure if they were even aware that it was the wrong method. When it started, they might have thought it was correct, and that thought was then passed down, so it is possible that they did it without questioning why," Mitsubishi Motors President Tetsuro Aikawa told a news conference at the transport ministry.

The automaker was ordered by the transport ministry to investigate the matter and report back by Wednesday.

The unapproved testing method — known as a running test — is one that is used in the United States. It helps to save time, but Japanese regulators have never approved its use.

Mitsubishi said it compared the test's outcomes with those of the approved Japanese test in 2001 and found barely any discrepancies. The difference was a matter of 2.3 percent, at most, it said.

The firm said it will compare the two methods once again, adding that it is not sure how many cars received the unapproved tests.

Regarding a separate scandal involving massaged fuel efficiency figures for more than 620,000 minicars, Mitsubishi said it is still investigating what happened and why. Four car models were involved.

The firm repeatedly set higher fuel efficiency goals for its minicars, and this may have pressured employees not to let the team down, Mitsubishi executives said.

For 157,000 units of the eK Wagon and eK Space models sold by Mitsubishi and 468,000 units of Dayz and Dayz Roox supplied for Nissan, Mitsubishi intentionally used figures for tire and air resistance that would be small on the road.

Mitsubishi stopped assembling and shipping the affected cars last week after the scandal came to light.

Analysts say the firm potentially faces huge costs for compensating parties involved, including car buyers, Nissan and the government — which granted it tax discounts for fuel-efficient cars.

The firm is scheduled to announce its earnings for fiscal 2015 on Wednesday but is likely to skip announcing its forecast due to future uncertainties.

Its shares crashed again Tuesday, closing the day's trading at the Tokyo Stock Exchange at ¥434, down 9.58 percent. The automaker's stock price has halved since the scandal broke a week ago.

Authorities raided the company's office last week after its initial admission of error and the firm has warned that the number of affected vehicles could rise. So far it has not included those sold overseas.

The analysts say the revelations raise questions about Mitsubishi's future and point to a broader problem in the global car industry as regulators probe other automakers' pollution and fuel-efficiency claims.

Embattled German carmaker Volkswagen said Friday the massive test-dodging scandal it is currently engulfed in pushed it into its first annual loss in more than 20 years, but that the final costs still cannot be estimated.

Also Friday, Germany's transport minister Alexander Dobrindt said a probe sparked by Volkswagen's emissions-rigging scandal found irregularities at 16 car brands, including Mercedes, France's Renault, Alfa Romeo, Chevrolet, Hyundai, Jaguar, and Nissan.

Information from AFP-Jiji added