More than half the UK's beaches are recommended for having excellent water quality in the latest Good Beach Guide - a slight rise on last year, the Marine Conservation Society said today.
The number of sites failing the MCS's latest bathing water tests, which are based on European standards, fell from 66 in the previous guide to 41 this year.
But around one in seven beaches, including tourist hotspots such as Rock in Cornwall, Weston-super-Mare's main beach and Robin Hood's Bay in North Yorkshire - are likely to fail tougher new EU standards being introduced in 2015, the MCS warned.
This year 421 bathing spots (55%) were given the thumbs up in the guide, an improvement on 388 (50%) last year.
But the conservation group said the latest tests, carried out from May to September 2009, showed the water quality of UK beaches had not returned to the record highs of 2006 because of extremely wet summers since then.
Higher seasonal rainfall causes a network of sewer overflows to discharge raw sewage on to some beaches from an overloaded system, the MCS said.
The high level of rain also washes pollutants such as animal waste, fertilisers and rubbish from farmland and cities into rivers and to the sea.
According to the Met Office, 2007 and 2008 combined were the wettest summers on record, and levels of rainfall were 42% above average in summer 2009.
Rachel Wyatt, from the MCS Good Beach Guide, 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 and into the sea.
"The regional pattern to this rainfall means that some regions such as north-west England and Scotland fared worse in this year's guide, whereas others like the Channel Islands did markedly better."
The MCS is also concerned that more beaches will fail the water quality tests when stricter European levels are introduced in 2015.
The society only recommends beaches if they exceed the existing higher "guideline" EC water quality standards and are not affected by inadequately treated continuous sewage discharge.
In the guide, beaches which only achieve European mandatory minimum water quality levels receive a basic pass and those which do not make that grade are given a "fail" rating.
More than 80 beaches around England and Wales will fail to meet even minimum water quality standards under the more stringent EU regime without action to improve them now, the society warned.
The number likely to fail minimum water quality standards accounts for 14%, or around one in seven, of Britain's bathing beaches.
MCS coastal pollution officer Thomas Bell said the beaches which are likely to fail in the future all have long-term water quality problems - with the combined sewer overflows partly to blame.
He warned that in a recession, authorities may be tempted to take the "cheap alternative" to fixing the pollution problems the beaches face, and simply stop recommending them as bathing spots.
"Our main concern is that these 80 sites that have been identified as being vulnerable at the moment do get improved and the cheap alternative isn't taken, which is to take them off the list of official bathing sites," he said.
He said that there are at least 500 UK beaches which have combined sewer overflow pipes, which carry sewage out of flooded sewer systems and into the sea.
While in many places the pipes are not a problem, 45% of beaches were not recommended by the MCS because of pollution and the percentage is set to increase as the tougher standards are introduced, he said.
Christine Tuckett, Environment Agency bathing water spokeswoman, said: "Bathing water quality around England and Wales has improved dramatically over the past 20 years but we are constantly looking for new and innovative ways to do more.
"New standards that come into force in 2015 will set even more ambitious targets and we are using the latest technology available to help meet these goals. Everyone has a part to play in improving bathing water quality.
"As well as using new technology, we are working with local communities to further improve our bathing waters in the coming years."
Beaches at risk of failing new European standards:
Aberdyfi England & Wales
Ainsdale England & Wales
Aldingham England & Wales
Allonby England & Wales
Askam-in-Furness England & Wales
Bardsea England & Wales
Bexhill England & Wales
Blue Anchor West England & Wales
Bridlington South Beach England & Wales
Burnham Jetty England & Wales
Cemaes England & Wales
Clacton (Groyne 41) England & Wales
Combe Martin England & Wales
East Looe England & Wales
Fleetwood England & Wales
Fraisthorpe England & Wales
Hastings England & Wales
Haverigg England & Wales
Heysham Half Moon Bay England & Wales
Ilfracombe Capstone (Wildersmouth) England & Wales
Instow England & Wales
Littlestone England & Wales
Lyme Regis Church Beach England & Wales
Morecambe South England & Wales
Mothecombe England & Wales
Newbiggin England & Wales
Paignton Paignton Sands England & Wales
Paignton Preston Sands England & Wales
Par England & Wales
Plymouth Hoe East England & Wales
Plymouth Hoe West England & Wales
Porthluney England & Wales
Rhyl England & Wales
Roan Head England & Wales
Robin Hoods Bay England & Wales
Rock England & Wales
Saltburn England & Wales
Seaton (Cornwall) England & Wales
Silloth England & Wales
Skinburness England & Wales
Southend Chalkwell England & Wales
Southport England & Wales
Spittal England & Wales
St Annes England & Wales
St Annes North England & Wales
Staithes England & Wales
Swansea Bay England & Wales
Teignmouth Town England & Wales
Torre Abbey England & Wales
Walney Sandy Gap England & Wales
Weston Main England & Wales
Weston-super-Mare Uphill Slipway England & Wales
Ayr (South Beach) Scotland
Brighouse Bay Scotland
Cruden Bay Scotland
Dhoon Bay Scotland
Ettrick Bay Scotland
Heads of Ayr Scotland
Kirkcaldy (Seafield) Scotland
Largs (Pencil Beach) Scotland
Lossiemouth (East) Scotland
Luss Bay Scotland
Millport Bay Scotland
Portobello (Central) Scotland
Portobello (West) Scotland
Ballygally N Ireland
Ballyholme N Ireland
Newcastle N Ireland