More than a quarter of Britain's top-rated beaches should be stripped of their Blue Flag status, campaigners said today.
According to research by Surfers Against Sewage (SAS), 34 beaches are unable to comply with a requirement imposed by the Blue Flag programme to warn the public about raw sewage spilling into the sea.
Britain has 131 beaches which have passed strict tests to fly the flag - showing excellence in water quality and beach cleanliness.
But using the Freedom of Information Act, SAS found local authorities responsible for 20 Blue Flag beaches in England, two in Scotland, nine in Wales and three in Northern Ireland did not request real-time information on combined sewage overflow (CSO) discharge.
The campaign group said those beaches could not therefore meet a mandatory requirement to warn the public during and after emergency pollution events, such as sewage discharge from a CSO, which could pose health risks to bathers.
To tackle the problem, SAS highlighted a scheme pioneered at Porthtowan, a Blue Flag beach in Cornwall, where CSOs have been upgraded with technology to alert the authorities as soon as sewage washes into the river and sea, so they can in turn warn the public.
SAS campaign director Andy Cummins said: "This disturbing revelation questions the integrity of the prestigious Blue Flag Programme.
"At 34 beaches around the UK the Blue Flag can be flying and people could be in the sea bathing in sewage-polluted waters without warning.
"To ensure the Blue Flag isn't devalued SAS are urging Blue Flag's governing body to lower these 34 Blue Flags until they meet all the imperative criteria."
The Blue Flag programme is a worldwide initiative run by independent non-profit group the Foundation for Environmental Education.
A spokesman for Keep Britain Tidy, which runs the scheme in England on behalf of FEE, said: "At Blue Flag beaches water quality is monitored by the Environment Agency during the bathing season and beach managers are required to display the results on the beach signage so the public can see them and make informed decisions about whether to enter the water.
"We cannot be present on every award-holding beach every day, and therefore if evidence is presented to us that calls into question that the high standards required are not being met, then we will investigate and take the appropriate action - even if that means withdrawing Blue Flag.
"Since the awards for 2010 were announced in May we have reminded the beach managers of the Blue Flag criteria and their obligations to take the Blue Flag down when their beach is not meeting all the criteria."
John Summers, chief executive of Keep Scotland Beautiful, which administers the scheme in Scotland, said: "Keep Scotland Beautiful takes the issue of combined sewage overflow spills seriously and is investigating the claims made by Surfers Against Sewage.
"Scottish Water, the Scottish Environment Protection Agency (SEPA) and the local authorities already work closely with Keep Scotland Beautiful to ensure that the information provided to the public about water quality and short-term pollution incidents is accurate and up-to-date at the beaches.
"Beaches without awards do not provide any information about water quality to users, so Keep Scotland Beautiful would encourage people concerned about the quality of the water to check the latest sample results on the information boards and to continue using the fabulous resources that we have in our Blue Flag beaches."
A spokesman for Welsh Blue Flag co-ordinator, Keep Wales Tidy, said: "When there is a pollution incident local authorities take immediate action to let us and the public know.
"We insist that water quality is posted on the beaches and Blue Flag beaches have a beach warden."
No-one from Tidy Northern Ireland was available to comment.
Here is the list of the beaches named by Surfers Against Sewage:
Carbis Bay, Cornwall
Bigbury on Sea, Devon
Ventnor, Isle of Wight
Shanklin, Isle of Wight
Sandown, Isle of Wight
Joss Bay, Kent
Margate Main, Kent
West Bay, Westgate, Kent
Botany Bay, Kent
Ramsgate Main, Kent
Lowestoft South, Suffolk
Cleethorpes Central, Lincolnshire
North Bay Beach, Yorkshire
Whitby West Cliff, Yorkshire
Seaburn (Whitburn), Northumbria
Prestatyn Central, Denbighshire
Llanddona, Ynys mon
Rest Bay, Glamorgan
West Sands, Fife
Elie Ruby Bay, Fife
Castle Rock, County Londonderry
Portrush West, Antrim
Whiterock West, Antrim