Some EU states are side-stepping their obligations to keep problem beaches clean, the European Commission says.
Beaches have to meet standards set out in a 1976 directive
Some beaches with "chronic pollution problems" are being deleted from official lists so they no longer have to meet with cleanliness regulations.
A commission report on 19,965 bathing areas in 25 EU states said 96.7% of coastal beaches were safe, but 10% of beaches in rivers and lakes were not.
New member states' beaches tended to be dirtier than in older states, it added.
Beaches on countries' official lists of bathing waters have to meet the standards of the 1976 Bathing Water Directive.
These standards were met in 96.8% of coastal beaches in old member states and 83% of beaches in the six new member states that responded in enough detail for records to be compiled.
But the commission pointed out that water quality tended to improve once regulations were enforced.
EU Environment Commissioner Stavros Dimas said although bathing water quality across the EU was generally good, there was scope for improvement.
"I am particularly concerned at the tendency of some member states to remove below standard sites from their official list of bathing waters and/or ban bathing at listed sites rather than bring them up to standard."
The commission said these steps were often being used at sites with "chronic pollution problems".
"Such an approach is not in line with the directive and the commission intends to discuss this matter with the member states concerned," the report said
In Italy, for example, the percentage of sites complying with the mandatory values fell to 67.9% in 2004 from 70.6% in 2003.
The proportion of beaches where bathing is banned has now reached 31.5%. The commission said it would take this up with Italy.
Meanwhile, Spain deleted 10 coastal and four fresh-water bathing waters from their official lists during 2004 and the proportion of bathing sites de-listed since 1996 has now reached 40%.
Again the commission said it would raise the matter with Spain.