It is believed thousands of fish have been killed after 450,000 tonnes of storm sewage was discharged into the River Thames.

A clean up of the Thames was carried out this week, under the supervision of the Environment Agency (EA), after it was reported thousands of dead fish and other aquatic life were spotted in the river along with raw sewage following heavy rain last weekend.

Downpours in London on Sunday and Monday caused 250,000 tonnes of storm sewage to be released into the river from combined sewer overflows along with at least 200,000 tonnes of storm sewage from Mogden sewage treatment works in Isleworth.

The discharges, combined with the current warm, dry weather and low river flows, resulted in the low oxygen levels and the death of the fishes along a 1km stretch.

Steve Holmes, a member of Thames Anglers’ Conservancy, tried to help fish which lay dying by Barnes Bridge.

He said: “We had to wade along the shore kicking up the water to oxygenate it and put back all the fish that had beached themselves, torpedo-ing them in to try to shock them into fighting.

“We saw, when you include the fry, 50p size flounders and livers, literally thousands upon thousands of small fish gasping for air on the surface. Many bigger fish had beached themselves to breathe their last.”

More rain fell in 24 hours in the south-east of the country on Sunday and Monday than in the whole of March, April and May, with more than 30mm falling on west London.

Howard Davidson, EA director for the south-east, said: “This is a major sewage pollution incident which has caused the death of a huge number of fish.

“We are currently monitoring Thames Water’s clean up efforts and assessing the full impact but unfortunately we may never know the exact numbers of fish that have died.”

Martin Baggs, chief executive of Thames Water, said the company “very much” regretted the fish deaths and environmental damage caused by the storm sewage.

He said: “Incidents like this are clearly totally unsatisfactory in a modern capital city and we have a major programme of work under way to sort the problem out.

“In the west London area this includes a 50 per cent increase in treatment capacity at our Mogden sewage treatment works, which will be complete early in 2013, and the proposed Thames Tunnel.”

To report pollution incidents call the Environment Agency’s emergency hotline on 0800 807060.