A government agency denies failing to stop farmers from polluting the river Wye, a court has heard.

Anti-pollution charity River Action took the Environment Agency (EA) to court for a judicial review this week, claiming it is allowing farmers to release destructive levels of nutrients from chicken manure into the river.

Large amounts of manure are spread over farmland surrounding the Wye to help crop growth but an overabundance can lead to an increase of phosphorus and nitrogen in the soil.

When washed into the river by rainwater, the excess nutrients can cause prolonged algal blooms which turn the water an opaque green, harming plant and fish life.

The Wye is the fourth longest river in Britain and partly forms the border between England and Wales as it runs from central Wales to the Severn estuary.

A judicial review is carried out when there is a dispute over whether a public body has acted lawfully.

On the final day of the hearing at the Cardiff Civil Justice Centre on Thursday, Charles Streeten set out the case for the defence.

He dismissed the claimant’s argument that the agency was not taking action against those who have breached the regulations, stressing it was EA policy to work with farmers to fix any breaches in the first instance, with enforcement as a last resort.

“Immediate formal enforcement action may be taken where there is a significant risk of pollution,” Mr Streeten said.

“Otherwise, the approach is to seek to effect change through advice and guidance and only to resort to formal enforcement action where a more constructive approach has not succeeded.”

He described this as being “rational” and insisted it was a matter of judgment for the EA when it was appropriate to take legal action.

Ned Westaway, who spoke on behalf of the Department for Environment, Food and Rural Affairs (Defra) – an interested party in the case having set out the regulations and guidance the EA follows, also objected to the charity’s case.

He argued that the amount of nutrients needed to be spread on a field was a “question of judgment”.

He also stressed that Defra felt there was a need for “proportionate enforcement” that strikes a balance between minimising costs for the farming sector and delivering environmental benefits.

The court also heard that letters sent to farmers who had breached regulations were clear about the need to comply, with the word “warning” on top.

“Farmers know what this means when they see this, it means ‘buck up’,” Mr Streeten said.

But in his rebuttal, David Wolfe KC – who spoke on behalf of River Action – argued that these letters did nothing but confuse farmers, did not set out a timeline for enforcement and failed to state that legal action against them was possible.

“Nothing is said to this farmer about needing to change this practice, that tells the farmer they are not in line with the law,” he said.

He argued the letters only “encouraged the farmer to do better and nothing more”.

The judge, Mr Justice Dove, said he would make his written decision in due course but did not set a date.