Swimmers and surfers were forced to flee the sea at the weekend after raw sewage was found to be pumping into the water on the north coast.
Lifeguards were warned about the situation at Gwithian and Godrevy at lunchtime on Saturday.
They ordered the immediate evacuation of the sea by swimmers and surfers, effectively shutting down two of the county’s most popular beaches on a peak summer Saturday.
It also led to the cancellation of the 10 Board Challenge surfing competition, which was taking place at Godrevy.
Writing on its Facebook page, the Trelawney Alliance Campaign Group said: “This is not an isolated incident; it has happened twice so far this year and several times last summer.
Members claimed this was the result of heavy rain in the Camborne area forcing floodwater into the sewerage system, which was unable to cope, leading to the sluice gates at Kieve Mill having to be opened in order to release the overflow.
The group added: “We pay the highest water rates in the UK but it would appear that South West Water are doing very little to upgrade their inferior sewerage system presumably in order to appease their primary masters, the shareholders.
“It is a fact that the pumping station at Kieve Mill cannot cope; this is particularly alarming considering that there are another 4,000 new homes with planning consent in place for Camborne.
“This situation is totally unacceptable and we need to be asking South West Water what they are planning to do about it.”
Disgusted beachgoers also took to the page to complain about the state of the beaches.
Sheila Saunders wrote: “We were shocked when we went to Godrevy early afternoon. Looked like the red river pollution all over again! It's disgusting having this happen. What can we do about it?”
Paul St John agreed: “Absolutely disgusting, we were there yesterday. We were aware of the red flag when we got there. And what I thought was a sandbar turned out to be sewage.
“It’s a beautiful beach and area with lots of varied wildlife, such as seals. Now they have to hunt fish through that filth. I beg to question how a little rain overpowers drainage systems and runs off into the sewer system. Again a case of greed over common sense.”
A spokesperson for South West Water apologised for the cancellation of the surfing event, adding: “The recent heavy rainfall led to a number of permitted overflows in line with our consents.
“We take proactive steps through our online BeachLive service to alert beach managers to these unfortunate situations so they can act accordingly and beach users can make informed decisions as to whether to swim or surf. We apologise for the inconvenience caused.
“More than £2 billion has been spent helping to clean up Devon and Cornwall's bathing waters and an extra £20 million is being invested this year. Unfortunately, during periods of intense rainfall, the system can sometimes become overloaded. We continue to work with our partners and regulators to ensure that public health is protected.”
West Cornwall MP Andrew George is demanding answers from South West Water chief Chris Loughlin and has also taken it up with the Environment Agency.
He said: “This isn’t the first incident in the St Ives and St Ives Bay area. While we await further information about the reasons behind this leak, and of the potential impact, I am bound to ask whether South West Water have a sufficiently robust system in place to cope with the demands of what was unexceptional rainfall.
“If there’s a discharge of this sewage volume on this occasion with what appeared to me to be an unexceptional storm, then this has the potential to occur on a very regular basis. Mitigation measures will be necessary.”