SEWAGE is rapidly destroying a highly protected stretch of river in what experts are calling an “environmental disaster”.

Pollution is wiping out insect life in the Upper Avon between Amesbury and Stratford-sub-Castle, tests have shown.

The Salmon and Trout Conservation charity and Salisbury and District Angling Club have handed a scientifically-backed warning to the Environment Agency about the rapid destruction of the chalkstream, which was once bursting with life.

A three-year study found three sewage plants were a major source of harmful phosphates in the river.

Effluent discharges from these plants have been increasing “at an alarming rate” since 2015, says Nick Measham from the charity. “We have an environmental disaster on our hands and the river is being destroyed,” he added.

Tests show pollution has risen by 50 per cent in three years to more than double healthy levels.

And at the same time insect numbers have plunged by more than 90 per cent, which the charity said was “shocking”.

The number of mayfly in West Amesbury has fallen from 1,903 in 2015 to just four in 2017.

Mr Measham said this vital food source for birds and fish was being wiped out.

Angler Jan Szakaowski said sewage works at Upavon, Ratfyn and West Amesbury were an “unacceptable threat” to wildlife and this “unique ecosystem”.

“Unless something is done immediately there is an imminent threat that the damage will become even worse.”

They want the Environment Agency to force Wessex Water to upgrade its sewage works.

Wessex Water said it would be happy to meet members of the angling club to discuss the concerns raised.

A spokesman said: “We look forward to hearing from the Environment Agency and studying the findings presented by the angling club in more detail.”

“Wessex Water has invested more than £50 million in phosphorus removal in the Hampshire Avon since 2000, resulting in dramatic improvements in water quality within the catchment.

“We have considerable water quality and ecological data from the catchment, collected over decades, and since 2015 we have gathered high-resolution data sets which capture influences from our assets and other sources such as septic tanks, urban and agricultural activities.

“This data is being shared with the EA and Natural England, and we would be happy to share the results with the angling club and discuss them in more detail if required.”