Beachgoers have spoke of their “disgust” after raw sewage was pumped into the sea on the north coast at the weekend, prompting the evacuation of the water.
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 said: "We're sorry that organisers cancelled the surfing event at Godrevy at the weekend.
"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 that £2billion has been spent helping to clean up Devon and Cornwall's bathing waters and an extra £20million 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."