A Surfer Against Sewage braves the sea in Cornwall while, below, one his companions makes a subtle point with a giant inflatable piece of faeces
There is nothing quite so reassuring as a blue flag flying when you unload a car full of kids down at the beach. The sand looks clean, there are not many bits of tissue paper clogging the shallow water. Not a turd in sight. Good, you think, it is safe to have a dip in the sea. Even so, you may look out for more information about the beach's cleanliness that might be posted on boards or at the tourist office.

But don't be too reassured. What you see is not necessarily what you get. Many of Britain's beaches are still polluted, despite European legislation, and a growing number of campaigners are out to change this.

"It's not a question of whether a beach is flying a blue flag or not, but of what kind of treatment the sewage is getting," says Chris Hines, director of Surfers Against Sewage (SAS), Britain's fastest growing environmental group.

On my own three-year-old's playschool trip to Walton on the Naze, Essex, I tested the theory. The water had been checked that morning: notice boards showed a green square for this beach (EC pass), blue squares for others nearby (EC blue flag). Despite this, many parents were reluctant to let their children in the sea.

I did, and three days later he had his first fever and ear infection in at least a year. Pure coincidence?

"Swimming activities in coastal areas where faecal pollution is present carry a real public health risk," the World Health Organisation concluded 10 years ago. Batch after batch of scientific evidence since have pointed to the very real risks, particularly for children, of anything from stomach upsets to hepatitis A and B after swimming in a sewage-ridden sea.

It is not that the problem is new. Sewage has been discharged into rivers and seas for centuries; the Victorians introduced sewage systems; by the late Forties a polio and typhoid outbreak was linked to sewage in swimming waters. And there cannot be many adults who never encountered a rusty old sewage pipe when they were roaming beaches as children. The difference today is that there is more sewage, we know its risks, and we have the technology to do something about it.

The meteoric rise of the Cornwall-based Surfers Against Sewage, set up five years ago by Mr Hines and five fellow surfers and with almost 20,000 members today, points to both the scale of the problem and the number of people who, as they frequently say, are fed up with swimming in diluted shit.

Surfers Against Sewage is one of many campaign groups. Travel the length of Britain's coast, some 3,000 beaches in all, and you will find any number: Save Our Seas in Fleetwood; Sons of Neptune in Scarborough; Friends of Cardigan Bay in Aberystwyth; the Northlea Action Group in Norwich.

All have similar tales to tell. "Going into the sea winter and summer, you see the changes taking place: the lack of fish, crabs, starfish; the clouding of the water, the deterioration in sea quality," says Freddie Drabble, leader of the 13-year-old Sons of Neptune.

His group sent a 20ft toilet down the Thames to the Houses of Parliament, a stunt that earned Mr Drabble, a solicitor, a spell in police cells.

The groups tend to campaign around local issues, with a persistence that is not always appreciated by water boards or local councils. Accusations tend to fly both ways.

One water authority, according to a campaigner, is a "Genghis Khan"; he says the authority is running an "agenda of misinformation". Fleetwood is becoming Blackpool's "new toilet", says another. But "we can't sort out a Victorian legacy overnight" was the retort.

Emotions are high because there is still an estimated 30 million gallons (two-and- a-half buckets per person) of raw sewage being discharged every day. And there are claims that new investment is being misspent. A pounds 500m investment programme includes a 3km vented tunnel which will carry sewage along the Blackpool coast to Fleetwood. It will let out a stench along the promenades and overflow directly into the sea in times of heavy rain, insists William Fairclough, a retired oil industry engineer and chairman of Fleetwood's action group.

The reality is that 10 years after the EC Bathing Water Directive gave water boards notice to clean up their act, almost one in five of the limited number of beaches monitored is still failing basic tests, according to the approval body, the National Rivers Authority (NRA). But even those that meet the minimum standards on bacteria are not considered hygienic enough by many.

"In North America, if the stricter guideline standard is exceeded, beaches are closed to the public on grounds of the health threat," says Pat Gowen, head of the Northlea Action Group. "Here we monitor 457 beaches, and not a single inland river, lake or broad."

There are no simple regulations for sewage disposal and, possibly, no simple answers: but whether sewage gets preliminary (macerated, then sieved through a 6mm mesh), primary (sludge removed), secondary (aeration) or tertiary (chemical) treatment depends on tentative facts such as whether the beach is a popular one, or whether the tides are such that raw sewage would be carried out to sea.

The NRA is confident that water quality will be much improved by 1997 as water companies, under consumer as well as increasing legislative pressure, step up their pace of improvement. It is no longer just active campaigners who are showing unease, but bill payers and holiday-makers, too.

While we are waiting for a proper clean-up, we can all help to speed up the process by voting with our feet. Do not give your holiday business to polluted beaches, and write to your MP, MEP, the NRA, council, water authority or local campaign group with your complaints. Evidence, or the lack of it from consumers, is noted in reviews.

Anyway, there will be no more fevers for my family. This summer we will be relishing the cleaned-up sea in Jersey or Aberystwyth.