From a wooden jetty, Cédric Giroud gazed out over the wide bend of the river Rhône, a picturesque, dark blue expanse dotted with swans. "At midnight on summer nights, when I'd finished fishing and boxed up my catch, I'd slip into the water and swim in the moonlight," he said.

The swell in the Rhône at the Grand Large just outside Lyon draws tens of thousands of French tourists on holiday weekends. It is a haven for rowers, sailors, fishermen and children feeding ducks. But under the crystal clear water lurks an environmental disaster the conservation group WWF is calling "a French Chernobyl".

The French government has banned the consumption of fish from the length of the Rhône - where it enters France from the Swiss Alps all the way down to the Mediterranean - after local specialities such as bream, pikeperch, carp and catfish were found to contain high levels of the toxic chemicals PCBs. France's second longest river has contaminated sediment in its bed and feeding fish have sent the toxins through the food chain. Environmentalists say the poison Rhône, which flows through tourist spots such as the papal city of Avignon down to the Camargue delta, is the tip of the iceberg of French industrial pollution, which the government has recklessly ignored for 20 years.

Freshwater fishermen talk of being suicidal. Local mayors and authorities have filed dozens of court cases after decades of campaigning by environmental groups. Research outside France has shown that PCBs - polychlorinated biphenyls once used in electrical generators, transformers and insulating fluid - cause infertility and birth defects in mammals. But the French government has not tested the toxic compounds' impact or carcinogenic effect on humans. The WWF, backed by 300 doctors, is now lobbying the government to urgently fund its own tests on health implications.

The ban on consuming fish from the Rhône has been extended to other French rivers poisoned by PCBs: in Normandy the popular delicacy of eels from the Seine has been outlawed as well as fish from the Somme. Scientists predict more bans will follow. The Chernobyl comparison by the WWF comes not from the potential number of deaths of humans, but from successive French governments' attitude of ignoring what campaigners call "a ticking timebomb".

The poisoning was not uncovered by the state but by Giroud, who sells his catch to African, Asian and eastern European immigrants in the local markets of Lyon's grey suburbs and old industrial heartlands. Giroud, 35, is the only commercial fisherman on the Grand Large in Décines, bringing in 10 tonnes a year and selling it himself. His freshwater fish - cheaper than sea fish and often sold for €2 (£1.50) a kilo - is a staple for poor people in the suburbs. Chinese and Vietnamese customers would use it for traditional dishes. One Turkish father used to buy 10kg to 20kg a week.

Then in 2004 birds started dying around the Grand Large. Tests showed it was avian botulism. "Although there was no effect on my fish, customers who had seen dead birds were wary," said Giroud. "Off my own back, just to reassure them, I sent my perfect-looking fish to the lab. I expected excellent results."

But the tests found a different, murkier poison - the fish contained PCBs between 10 and 12 times the legal safety limit. Fish from the Grand Large was banned at the end of 2005 and similar bans have progressively spread to other areas.

"This is the tip of the iceberg, the more research is done the more toxic contamination will be uncovered," said Alain Chabrolle of Frapna, a local environmental group which has campaigned on the effect of pollution in the river for decades. "There must be precise research on all possible PCBs sources, accurate maps and measures taken. The state polluted and allowed others to pollute. For decades they have put their head in the sand."

PCBs, one of the most poisonous groups of industrial compounds, were produced globally in large amounts before their danger was understood. They are still present in some industrial and electrical equipment and are difficult to dispose of safely. Trédi, an industrial waste processing plant formerly owned by the French state, sits on the Rhône 15.5 miles upstream from the Grand Large. It was supposed to limit pollution but instead emitted PCBs into the water. The plant's new owners insist they have cleaned up. But environmentalists say other sources of PCBs, such as disused factories, must be assessed. The Rhône cuts through France's biggest concentration of chemical industries.

Although Nicolas Sarkozy's new environmental super-ministry has now taken up the matter of PCB river poisoning, Chabrolle said the ministries of agriculture and health were slow to get on board, despite the implications for food and health. The freshwater fishermen banned from selling fish have received no compensation. French professional river fishermen are few in number and not a vocal lobby. In comparison, their seafaring counterparts have huge political clout, and if marine fishing becomes affected there could be a major political row.

"If they don't like something, marine fishermen will instantly block a major port with 100 vessels," said Chabrolle. "What can Cédric Giroud do, block the Rhône with one boat?"

In a nearby Asian supermarket a Vietnamese mother said: "I've always been suspicious of the river fish here." But other elderly French locals said they would be prepared to keep eating Rhône fish. Giroud said potentially amateurs could still sell the odd catch on the black market. Some of his customers still call him asking for it.

At a fishing supplies shop Didier Lardon has had to expand his stock to knives and toy guns to stay in business. "My takings are down 20% to 30% and the older amateur fishermen have not even renewed their licences," he said. Three fishing shops near the Grand Large have already closed.

If tests on the effect on human health begin Giroud and his four children aged six to 14 would be prime samples. But having lost his business and livelihood, he does not want to know. "Every day I think I'm just happy not to have committed suicide, that I still have my kids and my wife hasn't left me. It's not myself I feel bad about, it's having fed the fish to my children all these years."