Beaches in some of the Westcountry's tourist hotspots face becoming no-go areas after water quality standards plummeted to their worst level in more than a decade.
Environmental campaigners have described the situation as "disastrous" and warned that resorts will be shunned by tourists if urgent action is not taken.
South West Water blamed the standards failures – provided in figures from the Environment Agency and including beaches at Exmouth, Mounts Bay and Plymouth – on exceptionally heavy summer rainfall triggering overflow systems.
But Andy Cummins, spokesman for the pressure group Surfers Against Sewage, said it simply wasn't good enough.
"Without a doubt, this is unacceptable," he said.
"We have beautiful bathing water and beautiful beaches in the West and we need our water companies to protect our assets.
"What is important to tourists and to local people who love the beaches is how clean the water is."
According to the figures provided by the Environment Agency, beaches at Bude Summerleaze, Budleigh Salterton, East Looe, Instow, Mounts Bay Wherry Town, Plymouth Hoe East, Plymouth Hoe West, Seaton in Cornwall and Shaldon failed to meet the basic standards this summer.
At Weston-super-Mare's Uphill beach, a 30-year unblemished record was breached this season.
In 2015, tougher European standards for water quality will swing into place and failing to make the grade will mean high visibility warning signs advising people not to swim must be prominently installed.
At present it is estimated that more than 40 beaches will fail the new standard, including Rock in Cornwall and Combe Martin in North Devon.
Figures from this year will go towards the first raft of assessments to be carried out for the 2015 start date and Mr Cummins said the effects were easy to predict.
"It could mean that big red and white warning signs will have to be prominently displayed which say the advice is not to swim here because it has failed water quality standards. That could be disastrous to tourism.
"You could see people coming down on holiday and then seeing that sign and turning around and never coming back.
"If there is a problem people deserve to know about it because there are some very nasty pathogens out there."
An Environment Agency spokesman said full water quality standards results for the summer would be published in November.
However, he said the organisation was not expecting a good year: "Some beaches that have never failed before have failed.
"We are bracing ourselves for a bad set of results," he said.
Malcolm Bell, chief executive of VisitCornwall, said the bathing water quality failures caused by a "one in a 100-years" heavy summer rainfall were "regrettable."
He added that the variety and proximity of other beaches in Devon and Cornwall would always offer tourists alternatives should they find themselves advised against swimming at their first choice.
However, he accepted that it could be "quite critical" if resort beaches continually failed to meet required standards.
A South West Water spokesman said they were committed to ensuring beaches pass the 2015 bathing water standards.
"This summer was the wettest on record for 100 years and bad weather can adversely affect bathing water quality when heavy rain impacts on urban drainage and agricultural run-off," he said.
"Heavy storms can also trigger the operation of combined sewer overflows in the sewerage system.
"In preparation for the revised bathing water directive we have already invested £1.7 million in partnership with the Environment Agency to investigate the different factors that can impact on bathing water quality."