A campaign group is taking legal action against the Government alleging it abandoned a pledge to regulate microplastics and toxic chemicals in sewage spread over farmland.

Fighting Dirty argue the Government acted unlawfully by scrapping a commitment to reduce the use of harmful substances that are being washed into rivers and may be working their way through the human food chain.

A study last year by researchers from Cardiff and Manchester found the UK may have soils containing the highest concentrations of microplastics in the world, finding between 500 and 1,000 pieces per square metre.

Treated sewage is routinely spread over farmland as fertiliser and is sold to farmers by water companies.

As a waste product of sewage treatment works, the sludge can contain microplastics filtered out of the sewage water to prevent the polluting substances from reaching waterways.

The campaigners say this practice ultimately achieves very little as the sludge is then spread onto fields where the microplastics and other chemicals contaminate the soil and potentially enter the food chain, with the rest draining back into the environment where it can damage wildlife.

Spreading is considered a more sustainable way of using the waste compared to incineration or landfill, though the campaigners say there is no regulation in place to stop pollutants being sprayed over fields.

They say the Environment Agency and Therese Coffey as Environment Secretary are taking a “do nothing” approach despite having committed in 2020 to introduce new rules this year.

Campaigner Georgia Elliott-Smith said: “Farmers are unknowingly being sold potentially highly toxic material to spread on their land, poisoning our soil, watercourses, and food, and we have no hope of a date when this situation will be resolved.

“It is unacceptable to be left in limbo like this.”

The campaigners say the rules around sewage sludge have not been updated since 1989, and that since the commitment to introduce new standards was abandoned this year, no new deadline has been put in place.

Campaigner George Monbiot said: “It strikes me as a classic example of an issue that almost everyone has ignored, which turns out to be more important than many of those over which we obsess.

“The total failure of effective regulation in this case suggests that there is little ecological difference between dumping raw sewage into rivers – as water companies routinely and disgracefully do – and spreading contaminated sludge over farmland. Worse in fact, as the sludge poisons the soil before seeping into waterways.

“The rules are at fault. By failing to update them, and by suppressing and ignoring the evidence of its own officials, the Government is in breach of its legal obligations to protect the living world and human health.”

An Environment Agency spokesperson said: “As the responsible regulator in England for the supply, treatment, storage and use of sludge, the purpose of our strategy is to enable its safe and sustainable use on land.

“This ensures water companies meet their environmental obligations while contributing to clean and plentiful water and soil that is healthy and productive.”