More UK bathing beaches have excellent water quality than last year, but pollution has worsened since 2006, the Marine Conservation Society says.
It rated 421 of 769 beaches as excellent - 33 more than 2009, but below the 505 rated highly in 2006.
It blamed heavy rainfall during the past three summers for overloading sewage systems and washing fertiliser and pollutants onto beaches.
It also warned that several popular beaches would fail EU rules from 2015.
The annual Good Beach Guide is published on Friday and lists results of water quality tests conducted from May to September 2009.
It has four grades: "MCS recommended" for the highest water quality and good sewage treatment; "guideline" for good water quality only; "basic pass" means the water passed the European Commission statutory minimum; and "fail".
The latest guide recorded fewer failures than last year - 41 as opposed to 66.
Water quality had steadily improved from 2001 to a peak in 2006, but has declined ever since, the marine society said.
Based on Met Office data, the summers 2007 to 2009 combined were the wettest period since 1914.
The society said this caused overloaded sewer systems to discharge raw sewage on to some beaches, and pollutants such as animal waste, fertilisers and refuse to be washed from farm land and city streets into rivers and 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 Isles did markedly better," a spokeswoman said.
Tougher rules coming
It warned that stricter EU regulations, due to come into force in 2015, will see 83 beaches fail even the minimum water quality standard unless improvements are carried out.
The list included Rock in Cornwall, Paignton Sands, Robin Hood's Bay, Bridlington South, Chalkwell Bay at Southend, Plymouth Hoe and the main beach at Weston super Mare, it said.
Sewer overflow pipes, which transport raw sewage into rivers and the sea, were a cause of concern, said the society's coastal pollution officer, Thomas Bell.
"MCS knows of at least 500 UK beaches with one or more of these pipes.
"There are many places where they don't cause a problem but 45% of tested beaches aren't recommended by MCS because of pollution.
"That percentage is set to increase because of tougher bathing water standards from 2015, and we think [overflow pipes] are partly to blame," Mr Bell said.