Who can file an opposition before the INPI and how long does it take?
Obtaining a patent in France: the opposition procedure

With the exception of the owner of the contested patent, any individual or legal entity may file an opposition, without having to demonstrate an interest in acting ( article L. 613-23 du CPI ). In the view of legal writers, the absence of any requirement to demonstrate an interest in bringing an action means that actions can be brought "underwater" via a third party, without disclosing his identity to the owner of the contested right, which can be an advantage.

On the other hand, in nullity actions before the French national courts, the plaintiff must demonstrate that he has an interest in bringing the action (see Bringing down an annoying patent