French police are investigating an allegation that the former archbishop of Paris sexually assaulted a woman who is under legal protection as a vulnerable person, prosecutors said.

Michel Aupetit, who unexpectedly resigned in 2021 after admitting to an “ambiguous” relationship with a woman in 2012, denies any wrongdoing, his lawyer said.

The police investigation of Aupetit was opened on the basis of information from the Paris archdiocese, the Paris prosecutors’ office said, confirming French media reports.

It was launched in late November 2022, on a preliminary potential charge of sexual assault on a vulnerable person, the prosecutors’ office said. The alleged assault took place several years ago.

Aupetit’s lawyer Jean Reinhart said the probe was triggered by a letter sent to the Paris archdiocese. The letter was then forwarded to prosecutors, an automatic procedure for handling potential abuse cases that Aupetit himself put in place when he was archbishop, Mr Reinhart said. Prosecutors then launched the police probe.

Mr Reinhart said Aupetit has not seen the letter, has not been told who wrote it or what specifically it contains.

“My client is flabbergasted, doesn’t know what this is about,” the lawyer said. “We are completely in the dark.”

Aupetit became Paris archbishop in 2018. Pope Francis quickly accepted his resignation in December 2021.

The pontiff subsequently said he accepted the resignation because Aupetit could not govern effectively after “gossip” about his relationship with a woman removed his “good name”.

Francis said there had been “lapses” with Aupetit involving sexual sins. He said they were not that serious and involved “some caresses and massages”.

Roman Catholic priests take vows of chastity.

Aupetit’s resignation piled more upheaval on the French Catholic Church that has been severely undermined by a long history of sexual abuses. A report in October 2021 estimated that some 3,000 French priests had committed sexual abuse over the past 70 years.

In 2020, the pope accepted the resignation of French Roman Catholic Cardinal Philippe Barbarin in connection with the cover-up of sexual abuse of dozens of boys by a predatory priest.

Other investigations are also under way. Last November, the prosecutor’s office in the southern city of Marseille opened a preliminary investigation for “aggravated sexual assault” against Cardinal Jean-Pierre Ricard, one of France’s highest-ranking prelates of the Catholic Church.

In a letter that was read out during a conference of French bishops, Ricard said that he had abused a 14-year-old girl 35 years ago and was withdrawing from his religious duties.