More than nine million adults in England are unable to swim, according to a survey released by the Amateur Swimming Association (ASA).
The study found that 20% of men and 22% of women aged over 14 in England cannot swim, with the highest number of non-swimmers aged above 65.
The report, which also suggested that 2.13 million adults want to learn to swim, calls on policy makers to ensure swimming is accessible to all.
The report also showed that swimming is the most popular participation sport in England, with more than 2.6 million adults doing 30 minutes of swimming at least once a week.
ASA chief executive Adam Paker said many adults suffer later in life after missing out on learning to swim at school.
He said: "There are many reasons why people may not feel confident in the water or are nervous about going to a pool, so we are working closely with community groups and local partners to identify and break down these barriers, and encourage the millions of adults who want to become a better swimmer to do so."