THOUSANDS of fish are feared dead following a huge release of sewage into the Thames at Isleworth.

At least 200,000 tonnes of raw sewage was discharged into the river from Mogden sewage treatment works over the weekend after heavy rain.

An additional 250,000 tonnes of untreated water was released from sewage overflows along the Thames. The Environment Agency (EA) this week revealed large numbers of dead fish, including bream and eels, had been spotted in the river following the discharges.

A spokesman for the organisation said the sewage, combined with recent warm, dry weather and low river flows, had resulted in low oxygen levels which killed the fish.

Staff from the agency have added hydrogen peroxide to the water and used oxygenation vessels to increase oxygen levels.

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. Discharges from combined sewers happen 50 to 60 times a year and can be caused by as little as 2mm of rainfall. On Sunday (June 5), more than 30mm fell over west London.

"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."

He added that an 8-10km stretch of the river, from Barnes to Chelsea, was believed to be worst affected.

Thames Water deliberately released the sewage to prevent it backing up into customers' homes.

Martin Baggs, chief executive of Thames Water, said: "We very much regret the fish kill and other environmental damage resulting from the recent discharges of storm sewage to the tidal Thames."

Thames Water is already carrying out work, due to be completed early in 2013, to boost treatment capacity at Mogden by 50 per cent.

The company claims its proposed Thames Tunnel, meanwhile, will prevent discharges by carrying sewage from across London to a treatment works in the east.

However, proposed construction sites for the so-called 'super sewer' in Fulham and Barnes have proved controversial and public consultation is still taking place before work can begin.

Members of the public can report pollution by calling the EA's emergency hotline on 0800 8070 60.