PONTOISE, France - French children's rights groups have expressed outrage after a 28-year-old man was charged with assaulting rather than raping an 11-year-old girl because investigators said the pair had consensual sex.

The victim's family has called for the father-of-two to be tried for rape, saying the girl only went home with the man because she was "paralyzed" by fear and "unable to defend herself".

The girl is said to have agreed to follow the man home from a park in Montmagny, north of Paris, on April 24.

He is due to face trial for sexual assault of a minor in February.

To go on trial for raping someone under the age of 15 -- the age of consent in France -- there would need to be proof that the man attacked the girl, threatened or constrained her, or thrust himself upon her without her expecting it.

But investigators said they had insufficient proof that the sex was non-consensual.

Campaigners say the law needs to be changed.

"The question of consent or its absence should never be asked for rape victims who are minors," children's rights group La Voix de l'Enfant said in a statement Tuesday.

Armelle Le Bigot Macaux, head of the Cofrade children's rights group, told LCI television: "It's truly terrifying to think that an 11-year-old girl is supposed to have the judgement of an adult."

The defendant said he thought the girl, with whom he had interacted twice before, was above the age of consent.

"The only question is whether my client knew the age of the plaintiff," said his lawyer Marc Goudarzian, insisting the sex was consensual.

He described the girl as "fearless" and said she had sent nude photos of herself over the internet.

The family's lawyer Carine Diebolt said she could argue there was an absence of consent, saying the defendant had committed violence against her and had threatened to ruin her reputation.

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© Agence France-Presse