Yesterday a spokesman for the Environment Agency said staff were at the scene monitoring oxygen levels in the water, which is crucial to fish survival.

A special oxygenating boat was being used to pump the life-giving gas into the water to to try to save other fish which had so fare survived.

The sewage flooded out into the river which is popular with canoeists and anglers, from the combined sewer at the Mogden Sewage Treatment Works in Isleworth.

Thames Water's reoxygenation vessel Vitality has been on the site since Friday evening and the company is also using its Hydrogen Peroxide station at Barnes in an attempt to boost oxygen levels.

“This is a vast improvement from what the teams were seeing yesterday,” said a spokesman for the Enviroment Agency.

“The crews have remained on the scene throughout the night and are continuing to monitor the situation.

“Obviously with more rain said to be on the way we will be watching carefully how things develop.”

Hundreds of dead fish were pulled from the water near the world-famous Royal Botanic Gardens at Kew.

The Environment Agency spokesman said that additional water was also being released into the river at Teddington Lock, would improve the freshwater quality further downstream.

Tanya Houston, an Environment Agency fisheries officer, said: “This was a very serious incident which has caused the death of hundreds of fish.

“Unfortunately we may never know the exact number 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.”

Susan Kramer, MP for Richmond Park and North Kingston, whose constituency borders the affected area of the river, said: This situation must be treated as a health emergency.

"People regularly swim and boat on this stretch of the river, and to have a sewage spill on this scale is disastrous. The spillage has already had a devastating effect on fish populations in the river, and the smell is very unpleasant for local residents and river users.

"Water bills are sufficiently high to allow Thames Water to put in place proper plans to prevent this kind of spillage."

Discharges from Combined Sewer Overflows (CSOs), which caused this particular leak, are said to be a regular occurrence by the Environment Agency and can happen following as little as 2mm of rain.

A spokeswoman from the Environment Agency said: "The Environment Agency has been a strong advocate to finding a solution to the Thames Tideway Combined Sewer Overflows (CSOs), and was part of the steering group that developed the London Tideway Tunnels solution.

"Once constructed by Thames Water, these storage tunnels will intercept the problem CSOs and should see the removal of significant discharges of storm sewage to the Thames, providing a much-needed upgrade to a sewerage system that has not seen major changes since early last century."

On the river in Brentford, West London, the untreated sewage was causing a stinking problem for angry local residents.

Alexander Roux, who is converting a boat on the Thames, said: “It is jaw-droppingly disgraceful and I have never seen anything on this scale before, not even in the Third World. It is just disgusting.

“There is faeces, condoms and sanitary products being pumped into the water yards from where children are playing.”

The 56-year-old who hopes to one day live on his boat said he now may be forced to re-think his plans.

“What I found was repulsive,” he said. “It is utterly disgusting that this can happen in this day and age.”

The River Thames, which was once so polluted that it was declared biologically dead in 1957, is now accepted to be cleaner than at any point in the past 200 years.

A spokesman from Thames Water added "Everyone else recognises that we can't just make sewage disappear.  It has to go somewhere, which is why these discharges are legal and why plans from Thames Water and the EA for a £150m project to provide 40 per cent more capacity have been approved by the London Borough of Hounslow".