Microbiologists, working for Holiday Which? magazine, dropped bread rolls buttered side down on to the sand and then transported them in sterile bags to a laboratory where they were tested for food poisoning bugs.
These tests identified what researchers described as "stomach-churning doses of E. coli bacteria" clinging to the samples taken from Blackpool, Lancashire, and Weston-super-Mare in Somerset. Three of eight samples taken from the beach around Blackpool's north and south piers contained high levels of E. coli and another two showed levels of the bacteria. Two of the four samples from the beach at Weston-super-Mare also revealed high E. coli levels.
Roger Lakin of Holiday Which? said: "The doses we found would be a significant health risk if found in food. The most likely source of the bacteria is sea water polluted with sewage."
Ten beaches were tested, including Bournemouth and Newquay, where lower levels of E. coli suggested that improvements could be made to minimise contamination risks.
The Environment Agency, which is responsible for testing bathing water standards, accepted that there had been problems with sea water quality around Blackpool. A spokeswoman said North West Water was investing pounds 100m to improve water quality in the area.
Dr Steven Gee, a specialist in communicable diseases, said holidaymakers were at more risk of catching E. coli from eating a "dodgy burger" than from sitting on the sand.