One of the UK's largest chicken producers and a water company will be in the High Court on Monday accused of polluting the rivers Wye, Lugg and Usk.

More than 4,500 people who live or work near the rivers along the Welsh-English border have signed up take part in what's being seen as a landmark case against Avara Foods and Welsh Water.

Their lawyers say it's the biggest case ever brought in the UK over environmental pollution in terms of the number of claimants and its geographical spread.

Avara, which dominates chicken farming in the area, and Welsh Water, have respectively called the claims "misconceived" and "misguided".

The River Wye is one of the UK's longest and most celebrated rivers. But in recent years those living nearby have complained that it regularly turns green in the summer and has become smelly and slimy.

The group's legal claim blames the spreading of chicken manure on farmers' fields, and also sewage spills. It demands that action be taken to improve the state of the rivers, and compensation be paid to those whose lives and businesses have been affected.

The case has its first procedural hearing at the High Court in London on Monday and Justine Evans the lead claimant will be there.

"That just isn't what this river should look like and feel like and smell like" Ms Evans, a wildlife filmmaker, told BBC News on the banks of the Wye, not far from her home. "There's been systemic failure going on. And so in light of that, it seemed like the only course of action is to take legal action and make polluters pay."

Campaigners have long pointed at the rapid expansion of industrial chicken farming near the River Wye. At present there are about 24 million chickens being raised in the catchment area – usually in huge sheds. That's about a quarter of the UK's entire chicken population.

Until recently manure from the Wye's chicken sheds was spread as cheap fertiliser on nearby arable farmers fields. The group legal claim alleges that the nutrients from the manure frequently washed off the soil into waterways leading to high levels of phosphorus, nitrogen and bacteria entering the rivers. In warm weather that nutrient load caused the water to turn green, so called "algal blooming".

In 2023, Natural England, the UK government's official advisory body rated the condition of the River Wye as "unfavourable - declining". The follow-up River Wye Action Plan in 2024 blamed excessive nutrients from farming and wastewater discharges as well as climate change for increasing the water temperature and reducing the water flow in hot dry summers.

Legal firm Leigh Day are bringing the case on a no-win no-fee basis. They say that that though it was arable farmers who spread the manure, Avara Foods and its subsidiary Freemans of Newent should be held responsible for the consequences.

"The poultry companies that are being sued in this claim knew what the outcome of their operations were going to be when they expanded the poultry production in this area," Celine O'Donovan, one of the Leigh Day lawyers told BBC News.

"As a result, the responsibility for the decline of these rivers needs to lie with the people that knew what was going to happen and have made the money from it and controlled the supply chain that resulted in it."

The companies being sued are accused of negligence, causing private and public nuisance and even trespass where the riverbed has been affected on a claimant's property.

In a statement Avara Foods said the allegations were "misconceived" and that it was "confident in our position and believe the claim is unsupported by any proper scientific basis." It said that river health is affected by "multiple factors" and that phosphorus levels had fallen since the early 1990s.

Welsh Water, which has been accused of increasing the nutrient load through sewage spills said the case was "misguided" and that it had invested £76m on reducing nutrient levels on the Wye, Lugg and Usk between 2020 and 2025 and would invest £87m more from 2025 to 2030.

For Nathan Jubb algal blooming is not just unpleasant and unsightly but a financial problem. He's what's known as a "gillie" and manages fishing along a stretch of the Wye – a river once famous for its Atlantic salmon fishing. Salmon is now said to be in a critical condition with just a few thousand migrating up the Wye each year.

Jubb told BBC News he's signed up to take part in the the claim and that though the number of salmon being caught has fallen dramatically in recent years he thinks the main issue is the green algae making them harder to find and catch.

"We don't know they're there because they don't usually show and we can't see them." he says, "People are just going away from the river, the anglers are just disappearing," he says with a sigh, "And they're not coming back. Because they're not catching anything."

Additional reporting by Tom Ingham and Gwyndaf Hughes

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