Mitsubish Motors has suffered a shares drop of over 15 per cent following the manufacturer’s admission that it falsified the fuel economy data of over 600,000 vehicles in Japan.

The Japanese car maker has revealed that tyre pressure figures were falsified by employees to flatter mileage rates. The inaccurate test are known to have affected 157,000 Mitsubishis and 468,000 vehicles which Mitsubishi produced for Nissan. The issue came to light when Nissan discovered inconsistencies.

The issue over fuel economy data affects a range of small city cars, commonly referred to as ‘mini-cars’ in Japan. Models affected include the ek Wagon, eK Space, Dayz and Dayz Roox, as well as two mini-cars made for Nissan. These mini-car models have barely sold in other markets but are hugely popular in Japan.

While only cars sold in Japan are confirmed to have been affected, Mitsubishi says they will investigate whether fuel economy figures were altered for any vehicles sold overseas.

Speaking in a press conference earlier today, the president of Mitsubishi Motors, Tetsuro Aikawa, said: “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.”

While Aikawa says he was unaware of the irregularities, he added that he felt responsible for what had happened.

In response to the news, the Managing Director of Mitsubishi Motors in the UK, Lance Bradley, 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."

The drastic drop in shares for Mitsubishi is the biggest fall the company has suffered in nearly 12 years.

This is the first time a Japanese car manufacturer has revealed a failure in undertaking fuel economy tests properly since the Volkswagen emissions scandal emerged during last year.