Allegations of emissions test cheating by French carmaker PSA Group are being investigated by prosecutors in France, according to reports.
Citing a court official, the Reuters agency reported that an official probe was opened into the maker of Peugeot and Citroen cars over suspected diesel test manipulation earlier this month.
It comes two months after PSA, which is 14% owned by the French state, was referred to prosecutors by France's consumer fraud watchdog DGCCRF.
PSA offices were raided by French authorities in April last year, and it became the fourth carmaker to be referred for suspected emissions cheating in the country after Volkswagen, Renault and Fiat Chrysler.
A PSA Group spokesman denied that the company's vehicles had been fitted with software to manipulate emissions during lab tests.
The spokesman told the AFP news agency that the firm would "defend its interests and those of its 180,000 workforce, its customers and partners" and that it "respects regulations in all countries where it operates".
French prosecutors decided to open an investigation into the carmaker after being warned that rigged emissions tests could "render its merchandise dangerous for human or animal health".
PSA agreed to buy General Motors' European unit, including Opel and Vauxhall, in an £1.9bn deal in February.
The acquisition saw PSA become the second-biggest carmaker in Europe after German group Volkswagen.
Carmakers have faced increased scrutiny over emissions testing after VW admitted in 2015 that it had falsified emissions data in 11 million of its diesel vehicles worldwide.
VW agreed a $15bn (£11.7bn) settlement with environmental authorities and car owners in the US last year over its emissions cheating.
It also agreed to pay a $4.3bn fine to US prosecutors in January this year – the largest ever fine imposed on an automaker in the US.
Six former Volkswagen executives and managers were also indicted by prosecutors over their role in the scandal, including three former heads of engine development.