A French investigation into alleged emissions cheating by PSA Group found suspect software had been used on almost 2m Citroen and Peugeot vehicles, news reports said.

PSA denied any use of fraudulent engine software, a spokesman told Reuters in response to a report in the Le Monde newspaper report.

Reuters noted PSA in February became the fourth car maker to be referred to French prosecutors by the country's DGCCRF watchdog over suspected emissions test-cheating, after Volkswagen, Renault and Fiat Chrysler.

PSA's engineering chief acknowledged at the time that emissions treatment in the group's diesels was deliberately reduced at higher temperatures to improve fuel efficiency and carbon dioxide (CO2) emissions in out of town driving, where NOx output is considered less critical, the agency noted.

According to Le Monde, an internal PSA document obtained by DGCCRF investigators includes discussion of the need to "make the 'defeat device' aspect less obvious and visible".

PSA insisted there was nothing fraudulent or illegal about its engine calibrations.

"PSA denies any fraud and firmly reaffirms the pertinence of its technology decisions," the company told Reuters.