More than 200 fish have perished after a sewage spillage from the Mogden treatment works.

Officers from the Environment Agency are today monitoring oxygen levels on the River Thames following the leak.

Heavy rain yesterday morning is believed to have led to sewage from a combined sewer overflow at Mogden Sewage Treatment Works in Isleworth to spill into the river. Most of the dead fish have been identified as flounder and were spotted around the Kew Gardens area.

Although more than 200 dead fish have been spotted there are likely to be many more which have died.

Tanya Houston, an Environment Agency fisheries officer, said: “This is a serious incident which has caused the death of hundreds of fish. Unfortunately we may never know the exact numbers of fish that have died.

“We have been working hard to monitor the situation and are taking action with the help of Thames Water. We will continue to do this as the incident develops.”

Additional water is being released into the river at Teddington Lock, to improve freshwater quality further downstream.

Oxygen levels are being studied at monitoring stations along the river and Thames Water’s reoxygenation vessel Vitality has been on site since last night.

Thames Water’s hydrogen peroxide station at Barnes is being used to boost oxygen levels, which are critical to fish survival.

The EA said discharges from combined sewer overflows can happen following as little as 2mm of rain.

The public can report pollution spillages by calling the Environment Agency’s emergency hotline on 0800 80 70 60.