"The entire chain of management" up to the French firm's chief executive Carlos Ghosn was implicated in the "fraudulent strategies", according to a report by anti-fraud agents which was seen by AFP yesterday.
Renault is part-owned by the French government, which holds just under 20 percent in the carmaker.
Renault shares were down 3.3 per cent in early afternoon Paris stock exchange dealings, at 79.48 euros, having early fallen by more than four percent. This added to the previous day's 3.7 percent drop.
Shares in fellow French manufacturer Peugeot were also down, by a much more modest 0.9 percent, in an overall firmer market.
"Renault doesn't cheat," Thierry Bollore, the French company's second-in-command told AFP by telephone late yesterday, saying all its cars complied with legal standards.
In a separate statement released today, Renault said "none of its services has breached European or national regulations related to vehicle homologations", or certification, and that it would fully cooperate with an ongoing official investigation launched in January.