Heavy summer downpours have caused bacteria levels to soar at many Westcountry beaches, which are now in danger of failing national standards as well as tough new European rules.
Conservationists say the rocketing levels of toxins – up 5,000% at some sites according to The Times newspaper – are due to drainage systems becoming overwhelmed and discharging raw sewage, including human excrement, sanitary products and condoms, straight into the region's best loved bathing spots.
Campaigners have warned that many of these key tourist areas are now in danger of slipping right back down the water quality league table, undoing years of hard work to shake off the unwanted label, "dirty man of Europe".
The Marine Conservation Society has warned of "an unprecedented risk to public health" this summer.
Pollution program manager Dr Rob Keirle said the results of awards schemes, such as the coveted Blue Flag, would look "pretty bleak next year" with many regular winners failing to reach the minimum standard.
And he said more exacting European standards due in 2015 would be twice as hard to meet, something which could seriously impact on key tourist centres and the economy.
"The rain has washed animal faeces and agricultural slurry off the land and out to sea – it's never been this bad before," he added.
But the pollution from natural drainage has been exacerbated by the effluent discharged into the sea by Britain's 22,000 combined sewer overflows (CSO).
Cornwall-based campaigners Surfers Against Sewage monitors CSOs at 50 beaches across Devon and Cornwall, sending out warnings through its pioneering real time alert system, run along with South West Water.
The group said the worst offenders, including around 20 in Devon and Cornwall, were discharging much more often than usual this year.
Campaign director Andy Cummins said around 100 beaches remain unchecked under the system, which could mean the figures were only "the tip of the iceberg".
He called for serious investment by water companies to prevent a slew of bad results.
"This is the first year of taking samples for the revised EU directive in 2015 – for the next few years we have got no slack – everything will have to work amazingly or we will have sewage warnings posted all over our beaches," he added.
"A lot of the EU doesn't have the CSO system we have – we have worked hard to shake off the 'dirty man of Europe' tag and we are on the brink of falling right back down the league table."
The Environment Agency, which tests bathing water, said "we should brace ourselves for pretty poor results".