OYSTER farming sites near the Fal estuary have been closed after contamination levels reached an all-time high.
The presence of E. coli in oysters at one of the three sites were recorded at 400 times the legal limit.
The scare comes less than two months ahead of the harvesting season and the Falmouth Oyster Festival.
For oysters to be farmed they need to contain less than 4,600 parts of E. coli per 100 grams of flesh.
According to recent tests by Cornwall Port Health Authority, the level recorded at Mylor Creek was 160,000 parts during the last monthly test on July 29.
At Mylor Pool it was a colossal 1.8 million parts.
At the third site, Grimes Bar in Malpas, Truro, the level was 16,000.
All three failed the test and oyster farming has been temporarily suspended.
But shellfish experts are at odds to explain the results.
Gary Cooper, from the county's Port Health authority said they were "disappointing" and results "out of character" with previous results.
He added: "We don't know what caused it. The Environment Agency and South West Water are working together to provide us with information to hopefully provide us with the source of the contamination."
Chris Ranger, from Cornish Native Oysters, who has 8,000 oysters in 'lays' in Mylor Creek, said there has only been one incident in the last seven years when oysters have not passed the contamination tests.
He said: "It is an unusual event. All we are hoping is that the Food Standards Agency realises it is either an extreme, isolated incident, or a mistake, and then discount the reading.
"If not then this is a threat to the industry."
He said the oysters he has stored in Mylor Creek can be cleaned with a certified purification machine, but the oyster season, which starts in October, could be affected if the shellfish do not pass the next tests in September.
