Geoff Sallis is one of 70 volunteers in the Safe Avon group who have tested the water in the River Avon more than 1,200 times between them.

The results show that phosphate and nitrate levels in the water have increased, which can lead to more algae growth.

Data from the Environment Agency shows sewage was discharged into the river at Tewkesbury for more than 5,000 hours over 228 days last year.

Geoff wants more to be done to monitor how much waste water companies dump into rivers.

“We need to prosecute companies that pollute the environment,” Mr. Sallis said.

“It bothers me that people are using these as open sewers for profit.”

In response, Severn Trent Water said all leak data was shared with the Environment Agency, adding: “We continue to be fully open and transparent while also reducing leaks and improving river health.”

The company also said it would invest £78 million in a river flushing project using pioneering ozone technology that will benefit Avon.

A spokesman for the company said: “We are ahead of government targets and leading the way in the UK.”