MP Susan Kramer is to meet Thames Water's chief executive this afternoon to press for answers after raw sewage leaked into the Thames from the Mogden treatment works.

More than 200 fish perished after up to 20,000 tonnes of sewage spilled into the river on Friday.

Heavy rain on Friday morning led sewage from a combined sewer overflow at the Mogden treatment works in Isleworth to leak into the Thames.

Environment Agency officers spent the weekend monitoring oxygen levels on the river.

Thames Water staff have also been working with the EA to boost oxygen levels, critical for fish to survive.

Ms Kramer, MP for Richmond Park, said today: “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 residents and river users.

“I am meeting with the chief executive of Thames Water this afternoon and will be demanding answers about how this was allowed to happen.”

The EA said discharges from combined sewer overflows can happen following as little as 2mm of rain.

A Thames Water spokesman said: "This legally consented discharge into the river took place following a very heavy downpour, which filled the storm tanks at Mogden, after which there was literally nowhere else for the storm sewage to go.

"We very much regret the impact on the river, but this was not an accident. The system is designed to work like this in such conditions.

"Had it not gone into the river, the excess flows would have backed up into people's homes nearby, and that wasn't an option.

"We dosed the discharge with hydrogen peroxide and injected oxygen into the River via our specially designed aeration boats (bubblers) to minimise the environmental impact.

"This incident shows why we and the Environment Agency are keen to increase the treatment capacity of Mogden sewage works.

"Hounslow borough council recently granted planning permission to extend the works. This will boost capacity by 40 per cent, making incidents like this far less likely."