Raw sewage has spilled from a flooded treatment works in Surrey after Storm Henk caused the River Mole to burst its banks.

Pollution spread over a public footpath beyond the fence of the unmanned Thames Water facility.

Heavy rain across southern England has led to the majority of Thames Water’s storm overflows discharging sewage into rivers and ground water.

It is likely that other water companies are also discharging more than usual but only Thames Water has published a live interactive map of its monitors on rivers.

Simon Collins, of the organisation River Mole River Watch, said the Horley treatment works near Gatwick Airport has flooded four times since Storm Ciaran hit the UK in late October.

He said progress to upgrade the works is too slow and that during a tour by Thames Water he saw sludge tanks leaking, in what he described as a “huge embarrassment” for the company.

After the flood on Tuesday he tested the water pouring through the fence and found phosphate – a pollutant found in sewage – to be “off the scale”.

Thames Water has apologised for the pollution and said it has installed a temporary pump to clear the water while it upgrades the treatment plant in the long term.

Mr Collins said: “What’s happening at the Horley sewage works really does show a lack of investment over many years.

“Every time the river rises to about 2.5 metres above the base level of the river, we know from my calculations that the storm tanks will not discharge into the river and they will simply fill up and overflow across a service road in the sewage works but also out of the sewage works.

“It pours out of the sewage works across a public footpath down a ditch – and it really is quite a torrent when it gets going – and it ends up crossing what is dog-walking territory, open public space, very close to West Vale housing estate.”

He added that as the facility is unmanned, it is up volunteers to monitor river levels and report any pollution.

A Thames Water spokesperson said: “We regard all discharges as unacceptable and are sorry to those affected by this.

“We have published plans to upgrade over 250 of our sewage treatment works (STW) and sewers including Horley STW.

“This will improve the sites’ ability to treat the high volumes of incoming sewage and reduce the need for overflows during wet weather.

“In the meantime, we have installed a temporary over-pump to help reduce flooding on the footpath and will be carrying out a clean-up of the area when water levels recede.”

The company did not say how it categorised previous pollution incidents at the works but Mr Collins said one previous floods was recorded by the Environment Agency as a category 4 incident, which means it had no impact.

Only categories 1, 2 and 3 are recorded as pollution incidents and used to work out whether a water company has met its targets in reducing spillages.

Mr Collins said: “This is clearly having an impact. And in fact, from our reading of these, it should be really at least category 3, if not category 2, because there are major offences to public space.

“It is clearly a contaminant. We tested it for phosphate and it came out off the scale.”

Thames Water’s live map shows the majority of its storm overflows discharging sewage across the region over Tuesday and Wednesday, with some having been discharging since early December.

Other water companies had promised to publish similar maps by the end of 2023 but none did, according to The Times.

A Thames Water spokesperson said: “Taking action to improve the health of rivers is a key focus for us and we want to lead the way with our transparent approach to data.

“We’re the first company to provide live alerts for all untreated discharges throughout our region, and this near real-time data is available to customers as a map on our website and is also available through an open data platform for third parties, such as swimming and environmental groups, to use.”