Almost 4,000 people in England and Wales are taking legal action over what they allege is six years of sewage pollution that has devastated three rivers, including the Wye.

In the largest environmental group action of recent times, 3,943 residents and business owners are seeking substantial damages from Welsh Water and the leading chicken producers Avara Foods Ltd and Freemans of Newent Ltd, alleging they are responsible for “extensive and widespread pollution” in the Wye, Lugg and Usk.

The group wants a court to order the companies take remedial action to clean up the rivers, which are supposed to be protected for their importance to rare wildlife, including otters, freshwater pearl mussels and Atlantic salmon.

Oliver Holland, the international and group claims partner at Leigh Day, who is taking the case, said: “We see this legal claim as the culmination of an extraordinary effort by local community members and campaign groups to research, monitor and advocate for their rivers.

“The claimants have watched their rivers become the subject of national press coverage, public campaigns, parliamentary debates and government pledges. Formally commencing this legal action is a concrete step towards protecting these vital national ecosystems.”

The firms being sued – Avara Foods, Freemans of Newent and Welsh Water – all deny the claim.

The Guardian revealed recently how pollution from chicken farms is devastating rivers including the Wye. As many as 23m chickens, a quarter of the UK’s poultry production, are raised in the river’s catchment area.

Sewage and pollution from poultry manure cause high concentrations of phosphorus and nitrogen in rivers, causing substantial growth of algae that cuts oxygen, suffocating fish and harming fauna, leading to the deaths of key species.

The leading claimant in the case, Justine Evans, a wildlife film-maker who has lived in the Wye area since 2012, said the river was once a beautiful place enjoyed by many people. She said: “This connection is something that should be nurtured, but instead it is being shattered by the worsening state of the river.

“My relationship with the river has completely changed in recent years, to the point I no longer feel comfortable going in it. It’s horrible to think what has happened to the wildlife it is home to. Kids are growing up seeing the river as it is thinking this is its normal state, but it’s just not what rivers should look like.”

The state of the Wye has become a symbol of the poor state of rivers in England and Wales, which are suffering from a toxic cocktail of pollution from sewage and agricultural runoff. No river in England is in a good biological and chemical state as a result of pollution. Salmon – a leading indicator of river health – have been devastated in the Wye with catches from the last five seasons being the worst on record.