Lawyers for one of the country’s biggest producers of industrially farmed chicken have attacked a claim that they are responsible for pollution in the River Wye and River Usk.

More than 1,300 people have signed up to sue Avara Foods, its subsidiary Freemans of Newent and the local sewage company Welsh Water for extensive and widespread pollution in the rivers and their catchment areas.

In what their lawyers are calling the UK’s biggest ever environmental pollution claim, they blame the companies for the rivers turning green in the summer and becoming smelly and slimy.

But at a preliminary hearing at the high court in London on Monday, Charles Gibson KC, representing Avara and Freemans, said the claim that their activities had caused the river pollution was “entirely inferential and is an oversimplification”.

In written submissions, he said: “Their claim is fundamentally misconceived in law and in fact, lacking in any proper scientific basis, and misunderstands how poultry farms in fact operate.”

The barrister said those bringing claims should set out how they had been personally affected and the approximate date it began.

He said: “In all of these causes of action, it will be critical for each claimant to establish not merely that some parts of the River Wye and its tributaries had been polluted, but that the claimant himself or herself was personally affected by that pollution, and that such pollution actually caused him or her actionable loss and damage.”

About 24 million chickens – about a quarter of the UK’s entire chicken population – are raised in the Wye’s catchment area, mostly in huge battery farms. The claim alleges that pollution has been caused by water runoff from farmland containing high concentrations of phosphorus, nitrogen and bacteria resulting from the spreading of thousands of tonnes of poultry manure from the farms, as well as sewage bio solids.

High concentrations of phosphorus and nitrogen in the river have caused substantial growth of algae, which cuts oxygen, suffocating fish and harming fauna, leading to key species deaths, as well as reduced growth and bad smells as it decays, according the claim.

Anneliese Day KC, for the claimants, said in written submissions: “As a result of pollution for which the defendants are responsible through their agricultural/sewage-related activities, the health of the River Wye, the River Usk and their tributaries has declined.

“The ecological decline of the rivers has caused harm to the claimants, who bring claims for substantial damages and injunctive relief against the defendants.”

She said 1,309 people had joined the claim so far, while about 300,000 people live in the Wye and Usk catchments and “depend on the rivers as a shared environmental resource”.

Judge Cook described the claim as an “omnibus” on which “anybody can get on board”. He continued: “I was quite frankly taken aback by how the claimants have gone about this.”

The hearing finished on Monday with a further hearing expected at a later date.

Additional reporting by PA Media