According to research from the Amateur Swimming Association (ASA), A third of children in England cannot swim by the time they leave primary school.
The research suggests many non-swimmers have never had a school swimming lesson despite its being part of the national curriculum for seven-to-11-year-olds.
The ASA says swimming is the only curriculum subject that saves lives.
The Department for Education said schools must provide lessons and pupils must be taught to swim 25m unaided.
The researchers for ASA and cereal company Kellogg's, sponsors of the association's swimming awards, set out to find what proportion of 11-year-olds achieved the national curriculum target of being able to swim 25m by the time they left primary school.
Only 35 local authorities in England - around a quarter of those contacted - gave full responses to Freedom of Information requests from the team, relating to their records for 2011.
Their answers revealed that two-thirds of children achieved the target, meaning one-third did not.