The Marine Conservation Society – publishers of the latest Good Beach Guide – said the drop in water quality was down to one of the wettest summers on record last year.
The rain and flooding led to an increase in bacteria and viruses in bathing water, coming from a variety of sources, including agricultural and urban run-off, storm waters, plumbing misconnections, septic tanks and dog waste.
In Cornwall, 51 of 82 beaches were "recommended" in the guide compared to 68 in 2012. Summerleaze beach, at Bude, which achieved the top rank last year, was among four in the county which failed to meet even minimum standards for water quality. None failed in 2012.
In Devon, eight beaches failed to meet the grade in the 2013 guide, compared to two in 2012, with the list including Instow, Mothecombe, Bantham, Shaldon and Exmouth. Of the 62 beaches tested in the county, 30 were "recommended", down from 45 in 2012.
The society's coastal pollution officer, Rachel Wyatt, said: "We have recommended fewer beaches in every English region, and in Wales and Scotland.
"In England, the North West and South West were particularly badly hit with the fewest number of recommended beaches for at least a decade.
"Action must be taken now. With stricter bathing water standards from 2015 and summers that appear to be getting wetter, the iconic image of people bathing off golden beaches could be at serious risk. There is no simple solution to sewage and animal waste reaching our seas.
"However, if the water industry, communities and local authorities recognise that there is a problem and begin to work together to find answers, that would be a significant start."
The conservation organisation said there was an urgent need for improved monitoring of overflow pipes which can discharge raw sewage into rivers and the sea from sewer networks when heavy rain overloads the system with water from street drains.
And action was needed to reduce pollution from farms and urban areas, ahead of the tougher EU rules on water quality. The society warned swimmers could fall ill from ear, nose and throat infections and gastroenteritis from bathing in polluted water.
Nationally, a total of 42 beaches failed to meet the minimum EU levels expected for bathing water in testing, a rise of 17, according to the Good Beach Guide.
Only 403 of the 754 UK beaches assessed were awarded the top "recommended" award for their water quality, 113 fewer beaches than previously.
The Marine Conservation Society said there were some promising local partnerships working together to identify problems and start trying to fix them, but in too many places there was an "out of sight, out of mind mentality" over water pollution.
The situation is a turnaround from last year, when a record number of beaches were given the top "recommended" award.
