One of France's most high-profile intellectuals and media personalities, Olivier Duhamel, is facing a criminal investigation after being accused of sexually abusing his stepson.
The accusation is contained in a book written by his stepdaughter, Camille Kouchner, which has been serialised before publication.
She says her twin brother was abused three decades ago, as a teenager.
Mr Duhamel, 70, described the allegations as "personal attacks".
The political scientist and constitutional expert has not commented directly on the claims in the book. But on Tuesday he resigned from his academic and media roles saying he wanted to protect the institutions he worked for.
Until now Mr Duhamel had been head of the National Foundation of Political Sciences (FNSP), a body that oversees the prestigious Sciences Po university. He also presented a show on Europe 1 radio and was an analyst on LCI TV.
Paris prosecutors said on Tuesday that they were opening an investigation into Mr Duhamel in connection with suspected rape and sexual abuse of a minor by a person in authority.
Camille Kouchner, 45, and her twin brother are the children of Bernard Kouchner - France's former foreign minister and co-founder of the Medecins Sans Frontieres (MSF) medical charity - and the academic Evelyne Pisier.
Mr Kouchner and Ms Pisier separated and she went on to marry Mr Duhamel. Ms Pisier died in 2017.
Camille Kouchner's book, La Familia Grande, has been serialised in L'Obs magazine and Le Monde and is due to be published on Thursday. She said the alleged abuse took place at the end of the 1980s.
"I was 14 years old and let it go ahead. I was 14, I knew and said nothing," she wrote.
In the published extracts she names her brother only as "Victor" to protect his privacy. Le Monde said that he had read the script and was happy for his sister to speak on his behalf. He told the newspaper: "I confirm that what my sister has written concerning the actions of Olivier Duhamel towards me is correct."
Bernard Kouchner, in a statement released by his lawyer, said that a "heavy secret that has been weighing on us for too long has happily been lifted. I applaud the courage of my daughter Camille".
Camille Kouchner told L'Obs magazine that many of those in Mr Duhamel's and Ms Pisier's intellectual circle in Paris were aware of the abuse at the time.
"Of course, I thought my book might seem obscene because of my family's fame. Then I thought to myself, this is exactly what needs to be done," she said.
In a post on Twitter, Mr Duhamel said: "I am stepping down from my posts after being the target of personal attacks as I want to preserve the institutions in which I work."