Just days after storm sewage from Mogden spilled into the Thames killing more than 200 fish - the Environment Agency has admitted a further 900,000 tonnes has seeped into the river.

Following downpours last Tuesday night storm sewage from pumping stations and treatment works along the Thames, including Mogden, was discharged into the river, it has been revealed.

The discharge came just four days after 20,000 tonnes of untreated sewage spilled into the Thames from the Isleworth treatment works causing the death of 200 fish, mainly flounder.

Laura Dowsett, environment officer for the Environment Agency, said: “We have been monitoring the impact of the discharges using automatic water quality monitoring stations since the start of the incident and have worked in partnership with Thames Water to mitigate the effect on the river.

“Our fisheries staff found no dead fish or evidence of fish in distress when they assessed the impact from an agency vessel.”

A spokeswoman from Thames Water said: "Millions of tonnes-a-year of storm sewage going into the Thames is as unacceptable to us as it is to Londoners.

"The discharges over the past week draw into sharp focus the need for us to significantly boost treatment capacity at Beckton, Crossness and Mogden sewage works and to begin work on the proposed London Tideway Tunnels - work to help clean up the Thames by reducing the amount of storm sewage over-flows.

"Until this essential work is done, London's sewer network - which is Victorian - and other facilities will remain overstretched. This means that after heavy rain there is simply nowhere else for excess storm sewage to go, which is why these discharges, though regrettable, are legal and consented."

Hounslow Council last month approved plans by Thames Water to expand Mogden which will allow the sewage works to increase its capacity by 40 per cent.

Thames Water said the expansion would reduce the need for it to use storm tanks on the site preventing untreated water from spilling into the river.