The incident occurred at a petrochemical plant near Marseille.
Bathers have been warned to stay clear of a stretch of coastline in southern France after a highly corrosive chemical substance leaked into the sea.
The incident occurred at a petrochemical plant owned by polyvinyl chloride production giant Kem One in the town of Martigues, near Marseille.
The company told news agency AFP the incident was acknowledged at 01.50 am on Thursday morning "at a ferric chloride solution storage tank in the production workshop of the Kem One site of Lavéra".
This highly corrosive chemical, which is used in particular for the treatment of wastewater, turned into a brown substance as it flowed into the sea, forming a toxic layer of about six hectares at the Auguette cove.
Forty men from the Marseille fire brigade battalion (BMPM) and ten firefighters were sent to the scene, carrying out reconnaissance on the extent of the pollution with boats and drones.
The leak has now stopped, but local authorities have closed beaches at Laurons and Bonnieu and banned boating, fishing and swimming activities in the area between Ponteau beach and Cape Couronne in the Gulf of Fos for a minimum of 48 hours.
An investigation has been launched into the incident.
France's Minister for the Ecologic Transition, Barbara Pompili, said on Twitter that "any damage found will be repaired by those responsible".