Renault’s board has accused former chief executive Carlos Ghosn of “questionable and concealed practices”.
The French carmaker also said it would stop Mr Ghosn’s pension, thought to be worth €765,000 (£653,00) a year, adding that it reserved the right to bring action against him in the courts.
It is the first time Renault has publicly criticised its former star executive, currently on bail in Japan.
On Wednesday, he vowed to hold a press briefing next week to “tell the truth”.
Mr Ghosn ran the Renault-Nissan alliance until he was arrested in Tokyo and held in jail last year on claims of financial misconduct.
Renault had initially questioned Nissan’s allegations – which Mr Ghosn strongly rejects – but carried out its own internal inquiry.
The carmaker said the investigation had uncovered “serious deficiencies” over expenses incurred at the Renault-Nissan Dutch subsidiary RNBV and “violations of the group’s ethical principles”.
Renault said it had also “informed the French judicial authorities of potential issues concerning payments made to one of Renault’s distributors in the Middle East”.
The company added: “Renault also reserves the right to bring action before French courts, as and when more specific information involving breaches of Renault’s interests becomes available.”
On Wednesday, Mr Ghosn, whose revival of both Renault and Nissan earned him a huge reputation in the motor industry, took to Twitter with an announcement that he planned to hold a news conference next week.
“I’m getting ready to tell the truth about what’s happening. Press conference on Thursday, April 11,” the 64-year-old tweeted from the @carlosghosn account on Twitter,
The account was created this month. It was the sole tweet, posted in Japanese and English.
Although Mr Ghosn is under strict bail terms – including a ban on using the internet – his lawyer, Junichiro Hironaka, told reporters the tweet did not violate those terms.
According to Reuters, Mr Hironaka also said there had been no contact from Tokyo prosecutors, after Japan’s Yomiuri newspaper and other media outlets reported authorities were preparing a fresh case against Mr Ghosn over payments the carmaker made to a business partner in Oman.