The affected area of France runs from Sault-Brénaz in the Ain to Vaugris in the Isère.

Tests continue on further stretches of the river and it may well be that the ban will be extended. Some are suggesting the ban may well go to the outlet of the river on the Mediterranean Sea near Marseille.

The French river is reported to contain high levels of highly toxic Polychlorinated biphenyls (PCBs), a chemical compound previously used in the manufacture of electric transformers and capacitors.

Examination of fish showed levels of PCBs over four times the recommended safety levels.

The ban came as the trout fishing season opened, and local fishing groups are alarmed that it could last for several years.

The presence of PCBs in the Rhône River is not a new phenomenon, although this is by far the worst recorded incident. There have been problems with PCB contamination since the 1980s, when controls on discharges into the river were not as tight. In 2005 a ban on fish consumption was introduced in a branch of the river around the city of Lyon.

Experts have not yet established the cause of the pollution, but it is suspected to emanate from a specialist waste treatment plant at Sault-Brénaz. The factory denies breaching agreed discharge norms.

The sale of PCBs has been banned in France since 1987 as they are considered carcinogenic, and they are not degradable. Existing users have until 2010 to remove all PCBs from present use.