The importer, Pêcheries Nordiques, told a documentary by Swedish broadcaster SVT it imported 103 tonnes of the fish caught in the heavily-polluted Baltic Sea from Sweden between 2011 and 2012 without being found out.
Its director, François Agussol, said in the documentary the fish was then sold to large supermarkets such as Carrefour. SVT said another large supermarket chain, Intermarché, also purchased the fish.
After the documentary aired, Agussol told news agency AFP: “Nobody told us this was illegal,” adding that it acted in good faith, and that tests on the fish did not pick out the dioxin.
Pêcheries Nordiques, based in Echinghen, imports fish from Scandinavia and sells them to supermarkets, retailers and food companies.
Last month, Swedish authorities announced a fisheries exporter in the southern port of Karlskrona illegally exported 105 tonnes of Baltic salmon to France. It did not identify the companies involved nor specify when the export happened.
The Swedish documentary said the fish was also exported in smaller quantities to other EU countries such as Denmark, Germany and Great Britain.
In 2002, the European Union banned Sweden from exporting Baltic salmon to other EU countries because of high levels of dioxin.
Sweden, Finland and Latvia negotiated an exception allowing the fish to be caught and sold within those countries, on the condition that consumers are warned about dioxin.
The Swedish government recommends that children and pregnant women should not eat Baltic salmon more than three times a year.
Similar warnings are in place for herring, trout and other fatty fish caught in the Baltic sea.
Dioxins are polluting by-products of industrial processes such as smelting, paper bleaching and pesticide production. They accumulate within fatty tissue.
While dioxins are present in low quantities in most people, the World Health Organization says long-term exposure to high concentrations can cause damage to the immune system, skin lesions, liver damage and cancer.
In 2004, the former Ukrainian president Viktor Yushchenko claimed he suffered dioxin poisoning in a deliberate attempt to poison him while he was an opposition leader.
The incident left him with a disfigured face.