The River Wye has been downgraded after experiencing declines of important species like Atlantic salmon and white-clawed crayfish.
Conservationists are concerned at Natural England’s updated status of unfavourable-declining and said without urgent help and appropriate management, the river will never achieve a favourable or recovering condition.
Classified as a Site of Special Scientific Interest, the Wye begins in Wales and runs along the border with England to the Severn Estuary.
It is home to numerous intensive chicken units and livestock farms on either side of the border which, combined with sewage pollution, has caused phosphate-fuelled algal blooms, The Wildlife Trusts said.
Algal blooms consume so much oxygen in the river that other forms of life struggle to survive.
Natural England last assessed the Wye and its tributary the River Lugg in 2010 and on seven units of assessment found one to be favourable, with the rest unfavourable-recovering.
It said that because of the decline in salmon and crayfish all assessment units must now be ranked as unfavourable-declining.
Joan Edwards, director of public affairs at The Wildlife Trusts, said: “That the Wye is in even worse condition now will come as no surprise to the people that love and live near it.
“But this new admission represents a shocking failure by the agencies and authorities in Wales and England that are supposed to protect this once beautiful river.
“Wider research shows that farm pollution is the main cause of its decline – that’s why the authorities must enforce the law wherever the causes of pollution are clear.
“It’s time to prevent more chicken sheds from being built and ensure that all farmers are rewarded for nature-friendly, cleaner food production methods.”
The group is calling for a moratorium on any new extended intensive livestock production units in the Wye catchment, publication of all water data held by Natural Resources Wales, the Environment Agency and local authorities, and for farmers working more sustainably to reduce pollution to be rewarded.
Jamie Audsley, chief executive of Herefordshire Wildlife Trust, said: “Current approaches have failed to keep the River Wye in a healthy condition.
“What we now need to see is a cross-government plan to bring the Wye back into a favourable condition.
“The Wye should be a river where salmon and otters thrive and people can safely swim.
“The plan will need to involve governments, regulators, farm businesses and others, and ensure a consistent approach across England and Wales.”
Environment Secretary Therese Coffey met with farmers, conservationists and local officials in the Wye catchment on Tuesday to discuss the pollution.
She said she wants to double the funding available for slurry infrastructure and for a scheme offering one-to-one advice to farmers on how to reduce water and air pollution, as well as grow the Environment Agency’s agricultural regulation workforce and pay farmers to work sustainably.
After chairing a roundtable discussion, she said: “The River Wye is clearly struggling and it is vital that we turn the tide on its decline.
“As I set out in our Plan for Water, we need local plans catchment by catchment, community by community, to tackle issues that are affecting water quality.
“Bringing people together from the local communities, it is clear we have a common goal.
“We do all need to work together at a greater pace and with purpose to actively support our farmers and food producers to produce food sustainably and reduce pollution.
“With the River Wye rising in Wales, it is important to work with the Welsh Government on this issue and I welcome them joining my roundtable today.”