France will file a legal complaint on Friday against Iran at the International Court of Justice for violating the right to consular protection, foreign ministry spokesman Christophe Lemoine said on Thursday.
Two French citizens, Cecile Kohler and her partner Jacques Paris, have been held in Iran for three years.
France is accusing Iran of keeping them in conditions akin to torture in Tehran's Evin prison and not allowing proper consular protection.
Iranian officials deny these accusations.
Speaking to reporters, Christophe Lemoine said Paris would file a complaint with the international court, which is based in The Hague.
The couple's arrest in May 2022
French officials have toughened their language towards Iran, notably over the advancement of its nuclear program and regional activities, but also the detention of European citizens in the country.
Iranian state television had aired a video some time after the couple's arrest in May 2022, with them appearing to confess to acting on behalf of French intelligence services, something categorically denied by Paris.