The company confirmed that fuel efficiency tests broke Japanese rules in at least 600,000 of its vehicles, of which 157,000 part of its own-brand light vehicles and 468,000 were vehicles produced for Nissan.
The affected models are the Mitsubishi eK Wagon and eK Space, and the Nissan Dayz and Dayz Roox, which are manufactured for Nissan by MMC.
Mistubishi has said it has immediately halted the production and sales of the four cars, which are only available in Japan.
At a press conference in Tokyo yesterday, Mitsubishi president Tetsuro Aikawa told reporters the test data manipulations had emerged after Nissan alerted the company to inconsistencies in its results, which made fuel efficiency in the vehicles look more favourable than it should have been.
"The wrongdoing was intentional. It is clear the falsification was done to make the mileage look better. But why they would resort to fraud to do this is still unclear," Aikawa said at the press conference.
Lance Bradley, managing director of Mitsubishi Motors in the UK, said: "Mitsubishi Motors Corporation has acted quickly and decisively. We understand that such issues are of great concern to consumers. I would like to reassure everyone that there is no evidence to suggest that UK or European models are affected."
In a statement, the company said it had "acted quickly" to apologise to its customers and stakeholders, and plans to put in place a committee of external experts to "thoroughly and objectively investigate the issue".
Initial reports of the fuel economy fix yesterday sparked the largest one-day fall in its share price for 12 years. Mitsubishi shares had lost 15 per cent, around $1.2bn (£837m), by the close of trading yesterday. Shares closed down 1.12 per cent lower on the Tokyo Stock Exchange today.
"Mitsubishi has found itself in hot water over these new allegations but it may not face a doomsday scenario like the one playing out at Volkswagen. The company will of course take a hit to its reputation at a time when consumer trust in automakers is already eroded.
"Even though the issue is one of fuel economy rather than emissions, in the eyes of consumers and governments, it is also likely to find itself tarred with the same brush as Volkswagen after its emissions scandal," Tom Glendinning, autos analyst at BMI Research, said.
"Like Volkswagen, Mitsubishi has also opened itself up to the possibility of extensive class actions from disgruntled consumer groups, associated dealers and regulatory bodies. However, the company could also find itself fighting legal battles on a second front if its business partner Nissan resorts to more hostile legal action.
"However, in a best case scenario where Mitsubishi finds that the problem is only confined to its domestically-produced Minicars then the consequences will not be too damaging to company performance or force a change to long-term strategy," Glendinning added.
VW emissions scandal
Mitsubishi's announcement comes after its rival, the German car-maker Volkswagen, admitted last September to rigging 11m vehicles worldwide by using "defeat devices" to cheat emissions tests.
The debacle wiped more than £13bn off the car giant's value and prompted mass recalls across a number of its ranges.
In March, the company announced it would also need to recall all the electric vehicles sold in the US since 2014 (some 5,600 electric e-Golf cars) because of a software glitch that could cause the cars to shut down and fail.