Successive governments have failed to deal with the threat posed by spreading sewage sludge containing toxic chemicals on farmers’ fields, a former chair of the Environment Agency has told the BBC.

About 3.5 million tonnes of sludge – the solid waste produced from human sewage at treatment plants - is put on fields every year as cheap fertiliser.

But campaigners have long warned about a lack of regulation and that sludge could be contaminated with cancer-linked chemicals, microplastics, and other industrial pollutants.