But the latest report said other rivers were in an even worse condition because of industrial dumping dating back decades, including the Seine which runs through Paris.

PCB was used primarily as cooling and insulating fluid for electricity transformers and capacitors, but has been banned in France since 1987 after research showed it could cause fertility, growth or cancer problems in humans.

"PCB has been very heavily used in industry since the 1930s, so we are suffering the consequences of long-standing pollution," said Nathalie Kosciusko-Morizet, the secretary of state of ecology.

She told Le Parisien daily she wanted to see the results of more tests before deciding about stopping fishing in the Seine, but held out little hope of a swift decontamination of any of the rivers involved.

"One cannot de-pollute all the Rhone. Technically and economically it would be impossible," she said.