According to Le Point magazine, an anonymous letter was sent to the head of RATP and to the six unions representing employees at the company detailing the sexual exploits of a man who went by the nickname 'the King'. The man concerned was a senior official in the UNSA union.
The letter, sent at the end of June, alleged that several female employees offered sexual favours in return for career advancement.
According to the letter, women "who refused to have sex had a hard life."
Men who witnessed the events were harassed by the perpetrators to make sure they kept quiet. "They were penalised in their careers because they disobeyed the demands of the King," said the letter.
The letter claimed the union's offices on the boulevard de Sébastopol were used for the sexual encounters, as well as several hotels around Paris.
RATP management said an internal inquiry was launched and the letter has been passed to the Paris prosecutor for further investigation. Le Point reported on Thursday that judge Roger Le Loire will lead the inquiry.
The union involved, Sud, has also lodged an official complaint against the man concerned for offences including sexual harassment and pimping. Newspaper Aujourd'hui reported the pimping charge relates to an offer made by the self-styled 'King' to the head of the Sud union, Olivier Cots, at the start of the year.
Cots told AFP that the 'King' had "benefited for several years from the complicity of his union and of management."
"Everyone knew about it," he said. "If anyone complained, it was made clear to them they should keep quiet."
A spokesman for the UNSA union, Frédéric Sarrassat, denied this, claiming they had "never been made aware of these incidents."
The 'King' himself was suspended from the union in early January for reasons not connected to the charges.
Aujourd'hui quoted various sources within RATP as saying that "salacious, even sordid" stories had been circulating for years, "but no one ever dared to give evidence because the person concerned had a lot of power."