Cobo and Pembroke were two of 41 British beaches which failed the tests
Two popular Guernsey beaches have been branded as failures by the Good Beach Guide.
Cobo and Pembroke Bay failed to reach the most basic standard of water quality set by the Marine Conservation Society who conduct the survey.
They tested the water quality at more than 750 beaches around the British Isles with 41 deemed to have failed.
Five other island beaches received recommendations from the tests conducted from May to September 2009.
Grandes Rocques, L'Eree, Port Soif, Saint's Bay and Vazon were among the 421 beaches praised for the excellent quality of their water.
A further six beaches, Bordeaux, Fermain, Havelet Bay, Ladies Bay, Petit Bot Bay and Portelet Bay, were given guideline or basic passes in the guide.
Richard Harrington, the society's communications manager, said the tests looked for levels of specific bacteria meaning the reason for the failures can only be sewage pollution or rainwater running off farmland.
Rachel Wyatt, Good Beach Guide officer, said: "In the last three years there's been a shift in the water quality trend on our beaches. From 2001 there was a steady improvement which peaked in the Good Beach Guide of 2006 when we recommended a record 505 beaches.
"Since then, water quality has declined due to high volumes of rain carrying storm pollution from the sewer system, farmland and towns into the sea."
Members of the society have raised concerns that the results may get worse when stricter bathing water standards are introduced in 2015, which would see a further 83 of Britain's beaches fail the new minimum water quality standard.