Thousands of fish were asphyxiated in the Thames at Wandsworth last week when almost two million tonnes of raw sewage poured into the river.

Once again London's Victorian sewage system was overhelmed by heavy rain.

The sewage, which formed a visible slick along the Thames at Putney, caused oxygen levels in the water to drop to a dangerously low five per cent in the Thames - well below the 40 per cent threshold for fish survival.

A bubbler boat called Thames Vitality cruised along the Putney stretch of the river, pumping in oxygen in a bid to save the lives of some of the river's inhabitants.

An Environment Agency spokesman said Thames Water - responsible for Wandsworth sewage treatment - was not to blame.

He said: "It is not strictly their fault. It is basically down to the Victorian technology. Whenever you have heavy rainfall, the system is designed to release any overflow into the Thames - which will prevent it overloading sewage treatment works and backing up into people's homes. This dates back to the 19th century."

Thames Water, the Government and Environment Agency are investigating funding for a £1.5billion through-channel to take excess effluence to spare treatment works in East London.

A Thames Water spokesman said: "We accept that this can't be sustained in the long term, and have worked closely with the Environment Agency to identify solutions, which have been submitted to Government.

"Defra has short-listed two options, both of which involve building a tunnel under the Thames. We're carrying out further studies at Defra's request to ensure they have the detail they need to make a final decision."

All the Wandsworth sewage came from a 150,000 tonne release at Mogden treatment works near Twickenham and a 200,000 tonne discharge at Hammersmith. There are around 52 smaller discharges each year.

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