The number of adults swimming at least once a week in England has fallen by more than three quarters of a million in the past decade – a 23.7% drop.
The drop in participation comes despite more than 500 new swimming pools opening across the country since 2006.
Sport England data shows 2.5 million people swam weekly in 2015-16, down 776,000 from 3.27 million in 2005-06.
The Amateur Swimming Association said more people were choosing activities such as aquacise and wild swimming.
Swimming, however, remains England’s most popular mass participation sport, with about one in every 20 people aged over 14 swimming at least once a week.