Farmers leaders are demanding a full investigation into claims that untreated sewage has been dumped in the River Usk.

NFU Cymru has written to rural affairs minister Lesley Griffiths to raise concerns about a BBC Panorama investigation which aired earlier this month.

The union wants a full investigation by the Welsh Government and Natural Resources Wales to assure farmers that “agriculture has not been held accountable for the pollution caused from other sources, particularly sewage treatment works”.

Panorama said data from its environment information requests showed three of Welsh Water’s treatment works had breached their permits.

But the company insisted it had not operated illegally and that it worked hard to comply with all regulations.

NFU Cymru president John Davies said the union had long highlighted that there was a range of factors influencing water quality in Wales.

“The evidence is clear that a sole focus on agriculture through the introduction of regulatory measures to tackle agricultural pollution will not deliver Water Framework Directive objectives, yet we have observed a false and flawed narrative developing in recent years that frames agriculture as ‘the problem’ with respect to water quality issues in Wales,’’ he said.

“In light of the evidence uncovered by the Panorama programme, NFU Cymru is seeking confirmation on what actions are being taken by government and the regulator to stop these illegal practices.

“We also ask what investigations are being undertaken to understand if and where else within Wales similar illegal practices are or have taken place.”