French authorities last month opened a preliminary inquiry into allegations that the former archbishop of Paris had committed "sexual assault on a vulnerable person", prosecutors said on Tuesday.

The probe was opened based on a report filed by the diocese of Paris, they said.

Michel Aupetit offered to resign in late 2021 following media reports of an intimate relationship with a woman in 2012 before he took on the post, allegations he has categorically denied. Pope Francis accepted the resignation.

French broadcaster BFMTV reported that the relationship was with a vulnerable person under judicial protection.

A source close to the case told AFP the probe was looking into "email exchanges" between Aupetit and the woman, whose consent would have to be confirmed in view of her mental health.

In a statement on Tuesday evening the diocese confirmed it had filed the report, and said it was "not in a position to verify whether the facts in question are proven or whether they constitute an offence".

The clergyman's lawyer Jean Reinhart refused to comment.

"We have absolutely no knowledge of a complaint, so we cannot comment on the subject," he said.

A diocese spokeswoman in 2021 said Aupetit "had ambiguous behaviour with a person he was very close to", adding that it was "not a loving relationship", nor sexual.

But she said the offer to step down was "not a confession of guilt".

Aupetit was archbishop during the April 2019 fire that ravaged the Notre-Dame cathedral in Paris, regularly appearing on television to express anguish over the disaster and rally funds for the rebuilding effort. 

Catholic priests are bound to celibacy under church doctrine and are meant to practise sexual abstinence.

The French church is still reeling from the publication in October 2021 of a devastating report by an independent commission, which estimated that Catholic clergy had abused 216,000 children since 1950.

The pope has had to deal with an avalanche of allegations of sexual abuse by priests since being elected in 2013.

French prosecutors are also investigating Jean-Pierre Ricard, a retired bishop made a cardinal by pope Benedict XVI in 2006, after he confessed publicly to "reprehensible" acts with a 14-year-old girl in the 1980s.

He was one of 11 former or serving French bishops accused of sexual violence in a shock announcement by the French Catholic Church in November.