The Hogsmill River was contaminated with sewage twice in one week at two points of the river.

On January 24, a storm tank in Epsom overflowed following heavy rain, spilling sewage and rainwater into the river to prevent it backing up into people’s homes.

And on January 27, sewage sludge from the treatment works in Kingston was released into the river after a spillage.

Polly Bryant from South East Rivers Trust said: “With the storm tank, that happened right up in the upper reaches of the river, so everything downstream would have been affected.

“There were no fish killed or wildlife in distress, but that’s because the storm tank has existed for years, and the poor water quality stops wildlife from flourishing there.”

Storm tanks overflowing is perfectly legal; the system was built when the population of London and north Surrey was significantly lower, so the risks to wildlife were lesser.

The Hogsmill is a globally rare chalk river – one of about 200 worldwide.

Ms Bryant said: “Most chalk rivers are found in southern England. We really should be doing more to protect them, or even restore them. We are working with Thames Water to look at what we can do, but it’s not an easy task.”

A spokesman for Thames Water said they were still investigating the Kingston spillage, and do not know what caused it.

He added: “We’re sorry a spillage from our Hogsmill site entered the river in late January, and we’ve launched an investigation into what caused it. We worked closely with the Environment Agency and the watercourse returned to normal in two days.

“Although unpleasant and undesirable, such discharges at Epsom are legal in storm conditions. We understand the local concerns and are currently looking at ways to manage flows better across this whole catchment.”