At least one in four of Britain's premier bathing beaches are failing to meet the strict requirements of their "Blue Flag" designation, freedom of information requests to local authorities and beach operators have found.

The result is that tens of thousands of bathers who believe they have been swimming in Britain's cleanest waters may have unknowingly been exposed to raw sewage, according to pollution watchdog group Surfers against Sewage (SAS). The beaches in question have no system in place to monitor daily sewage pollution or to warn people if an overflow occurs. SAS says they should be stripped of their status.

Only 131 beaches in Britain have been awarded the coveted Blue Flag status, an international standard that is only granted if beach operators meet more than 30 strict criteria. Local authorities, who compete to get the coveted designation, pay more than £600 a year to be allowed to fly the blue flag.

But SAS research seen by the Guardian shows that 35 of the 131 beaches cannot possibly meet criterion 28 of the Blue Flag code. This requires beach operators to warn the public during and after emergency pollution events, such as a sewage discharge. According to the FoI requests made by the group, many local authorities responsible for accredited beaches do not ask for any data from water companies on combined sewage overflow spills, where heavy rain causes sewers to flood and discharge into the sea.

The 35 beaches named today by SAS include some of the most popular in Britain (see full list below). There are 20 in England, including Polzeath and four others in Cornwall, Woolacombe and one other in Devon, Margate and four others in Kent and several on the Isle of Wight. A further nine beaches in Wales, three in Scotland and three in Northern Ireland were named.

Last night Andy Cummins, SAS director, called for the 35 beaches to lose their Blue Flag status. "It is a major concern that these 35 beaches could have the Blue Flag flying while the public could unwittingly be swimming around in raw sewage discharged from nearby combined sewer overflows. Pathogens associated with sewage polluted waters include ecoli 0157H, hepatitis A, and gastro enteritis.

"We have had many calls from people saying that they used Blue Flag beaches and who said they became very ill. It's impossible to prove that they have been made ill by pollution picked up there, but we have compelling cases of incidents impacting on people's health."

Cummins said he suspected that many other popular bathing beaches were regularly polluted by raw sewage: "There are more than 20,000 combined sewage outfalls (CSOs) and it is very hard to keep track of them."

The Blue Flag programme is a worldwide initiative run by independent non-profit group, Foundation for Environmental Education, based in Denmark. There are Blue Flag beaches in North, Central and South America, the Caribbean, Africa, New Zealand and throughout Europe.

According to its website, Blue Flag beaches and marinas that want to apply for the award must first be reviewed by a national and then an international jury. If the beach is awarded the flag, it is then visited at least once a year by a national controller and every few years by an international inspector.

In Britain, the national Keep Britain Tidy Groups undertake the preliminary inspections.

John Summers OBE, chief executive of Keep Scotland Beautiful, which is responsible for administering the scheme in Scotland, said: "Keep Scotland Beautiful is investigating the claims made by Surfers Against Sewage.

"Scottish Water, the Scottish Environment Protection Agency 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.

"Beaches without awards do not provide any information about water quality to users, so visiting a Blue Flag beach is recommended by Keep Scotland Beautiful," said Summers.

A spokeswoman for Keep Britain Tidy said it took the problem of sewage overflows "seriously".

"A Blue Flag is a sign of a quality, well-managed beach. 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," said the group.

"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. Blue Flag is a global scheme that has set procedures to be followed. Keep Britain Tidy and our partners follow all these. We also carry out spot checks during the bathing season."

First person: "Two days later I had a pus-filled eye"

The waves were small but perfectly formed, and the sun was glorious. Sitting out back on my surfboard, waiting for a decent set to roll in, all seemed well with the world. But two days later I would be sitting in my GP's surgery with a pus-filled eye and a raging sore throat.

Both of the Cornish beaches I surfed on a beautiful weekend in late May – Praa Sands and Porthtowan – have "excellent" water quality, according to the Environment Agency. Porthtowan also has a Blue Flag – the cleanest of the clean, supposedly. But Porthtowan also has a combined sewage overflow nearby and there's one not too far from Praa Sands, at Portleven.

Ducking through the clear blue waves on the way out , I saw no sign of "floaters", tampons and the other telltale signs of sewage. The rare offshore breeze was holding up the waves, making them "super-clean" in surf-speak, ie glassy and smooth.

But while it's impossible to link conclusively the hours I spent in the water with the conjunctivitis and inflamed throat that flared up within 48 hours, I am convinced those super-clean waves were hiding a very dirty secret. Damian Carrington

List of beaches

The beaches named by SAS as having no system in place to monitor daily sewage pollution or to warn people if an overflow occurs are:

Cornwall: Polzeath, Gyllyngvase, Carbis bay, Challaborough

Devon: Woolacombe, Bigbury on sea

Isle of Wight: Ventnor, Shanklin, Sandown

Kent: Joss Bay, Margate main, Westgate West bay, Botany bay, Ramsgate main

Suffolk: Lowestoft south, Cromer

Lincolnshire: Cleethorpes central

Yorkshire: Filey, North Bay beach, Whitby west cliff

County Durham: Whitburn north

Wales: Prestatyn central, Llanddona, Fairbourne, Abersoch, Aberporth, Llangrannog, Newagle, Langland, Rest bay

Scotland: Fife West Sands, Elie Ruby bay, Coldingham

Northern Ireland: Coleraine Castle rock, Portrush west, Whiterock west

• On 6 August this article was amended. The beaches in Dorset, and at Montrose and Porthtowan were mistakenly included in a list provided to us and have now been removed.