Footage shows a huge expanse of murky water at the beauty spot on the southwest coast.
The sea along part of the cove is clear blue, but the video shows much of it suddenly turning a dark colour.
The video was taken at Trevaunance Cove in St Agnes, a village on the north coast of Cornwall.
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South West Water said the storm overflow system - which pumps sewage into waterways before it is properly treated to relieve pressure on the network - was briefly triggered in the area on Sunday.
But a spokesperson said they believe other factors contributed to the murky brown colour of the sea.
The footage from the Cornishbeauty spot was captured by a surfer and initially passed to local media.
Amy Slack, from campaign group Surfers Against Sewage, said: “The shocking scenes of pollution discharging into the sea in St Agnes at the end of half-term weekend were sickening, but sadly unsurprising.
“Ineffective regulation and weak enforcement have led us to a position where 2.7 million hours of raw sewage were pumped into our waterways last year, poisoning precious ecosystems and making people sick.”
The group’s head of campaigns called for the government to “urgently” review its sewage action plan, “properly” fund the regulator and make efforts to “put an end to sewage pollution for good”.
Sewage has been pumped into the environment more than a million times since 2016, according to official figures released in August.
Around the same time, swimmers were being warned to stay away from around 50 beaches due to sewage discharges in the sea.
The government has unveiled a plan to tackle sewage spills in the UK with targets for water companies to make improvements by 2035. But this was called a “joke” and “licence to pollute” by opposition parties.
A South West Water spokesperson said the storm overflow system at St Agnes “triggered briefly” on Sunday following heavy rain.
“This was a short-duration spill and is unlikely to have caused the level of discolouration shown in the video,” they said.
“On this occasion, we believe there were other factors which contributed to the discolouration, such as mud in the water dislodged by the heavy rain flowing into the area from a nearby stream and runoff from agricultural land.”
The water company spokesperson mentioned how the climate crisis had a role to play in sewage discharges.
“As well as prolonged periods of extremely hot weather, we have seen heavy localised rainfall which hasn’t been able to permeate into the dry ground, meaning significant volumes run into our network, which can cause our storm overflows to trigger,” they said.
The Department for Food, Environment and Rural Affairs spokesperson said: “We have been clear that the failure of water companies to reduce pollution is totally unacceptable, which is why earlier this year we set out the strictest targets ever for companies to tackle sewage discharges in our waters.”
They added: “Where water companies are found to be breaching their obligations to keeping our waters clean, we will not hesitate to hold them to account.”