Pulmonologist Michel Aubier goes on trial on Wednesday for false testimony for having denied any links with "economic actors" when testifying under oath before a Senate committee on the cost of air pollution in 2015.
He went on to say that "the number of cases of cancer relating to respiratory illness [...] connected to pollution is extremely low".
If found guilty, he could be jailed for up to five years and fined 75,000 euros.
At a closed session of the same committee the next year he confirmed press reports that he was employed part-time by Total were true, putting his income from that source at 50-60,000 euros a year.
But that was a huge understatement, according to Le Monde newspaper, which has seen the legal inquiry's findings.
Aubier has been a medical advisor to the company since 1997, required to spend just nine half days a month on the company's premises, and his income tax returns show income from Total of 109,956 euros in 2014, 106,787 euros in 2013 and 99,402 euros in 2012, the paper says.
He has also been a member of the board since 2007, receiving shares worth a total of 61,548 euros in 2013 and 2014.
Conflict of interest
The investigators conclude that there is a clear conflict of interest between his employment by Total and his job in a Paris hospital, which earned him 68,112 euros in 2014, Le Monde reports.
Before retiring, Aubier, 69, was also a member of France's public health body and lectured at a Paris university, as well as often being interviewed by the media and working with a number of laboratories.
In this case, for the first time ever, the Senate has declared itself a plaintiff, and so have two NGOs, Ecologies sans Frontières and Générations Futures.
Aubier denies any conflict of interest because "he was not involved in industrial production", in the words of his lawyer, François Saint-Pierre.
His defence is also questioning the legal status of the Senate committee, arguing that it should not require an oath to be taken because the politicians sitting are not impartial judges.
The Senate inquiry found that air pollution costs France 100 billion euros a year in health costs, absence of workers and lower farm production.