Government guidance to farmers on following water pollution regulations may be unlawful, the environment watchdog has said.

In response to a complaint by WWF and ClientEarth, the Office for Environmental Protection (OEP) said it “reads as an attempt to create an exemption from, or a defence to a breach of” regulations in the Farming Rules for Water.

Such a failure to uphold the law would be serious, the OEP said, and it has written to the Department for Environment, Food and Rural Affairs (Defra) asking it to update the guidance as soon as possible and not wait until September 2025 when it is next due for review.

The watchdog said it was holding off on an investigation until it receives Defra’s response.

It found the Environment Agency may also have broken the law by misapplying the legal test and failing to consult with Natural England before allowing farmers to spread a larger amount of nitrogen on their fields as fertiliser.

Nitrogen, a key element for plant growth, is spread on fields as fertiliser either artificially or via animal and human waste.

Crops only absorb around 20% of the nitrogen spread however, the OEP has previously said, with the rest washing off fields into rivers where it can fuel algal blooms that choke rivers by consuming the water’s oxygen.

Fertiliser use also produces a greenhouse gas that is 300 times more potent than carbon dioxide – nitrous oxide.

Kate Norgrove, executive director of advocacy and campaigns at WWF, said: “Our rivers and the precious wildlife that live in them are dying and the Government and the Environment Agency are letting it happen.

“Nitrogen pollution – in particular run-off from farming – is continuing to cause untold damage to our rivers, driving air pollution and releasing greenhouse gases into the atmosphere.

“To turn things around for England’s rivers, we now need urgent action and investment from Defra to support farmers to protect river health, and to hold those responsible for pollution to account.”

The OEP said it will continue to monitor the Environment Agency and has asked it for further details to make sure it complies with legal requirements.

WWF and ClientEarth said the OEP’s view was “largely welcome” but that they are disappointed the OEP has not asked the Environment Agency to intensify its enforcement against polluters.

Earlier this month, the OEP found that the Government was “largely off-track” on its environmental targets, which include ensuring clean and plentiful water, managing exposure to chemicals and pesticides minimising waste.

Defra has been contacted for comment.