Last week, the firm said it had discovered data relating to fuel consumption of certain models had been misrepresented, and expressed "deep apologies" for the "improper conduct". UK PR professionals said the news, coming not long after a similar scandal engulfed VW, asked serious questions of the car industry.
Later that week, the UK Department for Transport published a report saying that while there was no evidence of manufacturers other than VW fitting 'defeat devices' to cars to get around emissions tests, there was "evidence that nitrogen oxide emissions are higher in real world conditions and on the test track than they are in laboratory conditions".
Mitsubishi today admitted that it had been supplying false data since 1991.
A statement on the company's global website said: "In connection with the certification process for the mini-cars manufactured by Mitsubishi Motors Corporation (MMC), MMC hereby notifies that, yesterday, the board of directors decided to establish a special investigation committee consisting of only external experts."
"MMC expresses its most sincere apologies to all of our customers, shareholders and stakeholders for any inconvenience or concern caused by this occasion," it went on to say.
The committee of three Japanese public prosecutors will review documents and interview Mitsubishi staff, analyse the cause of the falsification and make "a suggestion on measures to prevent recurrence". It will "investigate the Matter for about the next three months", before preparing a report.
"MMC plans to disclose the report in a timely manner," it says.
In sales and exports figures released today, the car firm said it had manufactured a total of 121,961 vehicles in March 2016 – with three fifths of those made in Japan – a slight decrease on March 2015.