Headteachers of leading girls' independent schools are fighting a rearguard action to keep their pupils.
They believe parents often have an outdated view of girls' schools and that they are won over by better-looking facilities in co-educational schools.
Latest figures show the number of girls going to mixed private schools rose by 9 per cent between 2001 and 2004. Numbers at single-sex schools rose by just 1.5 per cent.
The Independent Schools Council census shows that there were 45,772 girls in mixed Headmasters' and Headmistresses' Conference schools in 2001. This figure rose to 50,009 in 2004.
There were 104,501 girls in single-sex Girls' Schools Association schools in 2001, compared to 106,093 in 2004.
Hilary Moriarty, headteacher at Bedgebury, a private girls' school in Kent, said: "Girls do better in girls' schools, yet there is this drift away from single-sex schools. We have got to stop it.
"Boys and girls learn differently and you have to teach them differently."
But David Exham, general secretary of Woodard Schools, a 45-strong group of co-educational state and private Church of England schools, disagreed.
"Boys and girls learn from each other. Girls are hard-working and get the work in on time, but boys are risk-takers and that is immensely valuable for girls.
"Some girls do better in single-sex schools, but some do not and have a right to compete on equal terms with boys and show they can do just as well."
The lure of co-education does not appear to have affected state girls'
Highbury Fields, a girls' state school in Islington, north London, has been oversubscribed for the past 12 years and there are now around five applications for each place.
Bernard McWilliams, the headteacher, said: "The girls tend to gravitate towards single-sex schools. Even the co-ed schools here tend to be 70 per cent boys and 30 per cent girls."
Claire Oulton, head at Benenden, the Kent private girls' school attended by the Princess Royal, said that parents' vision of girls' boarding schools was often 20 or 30 years out of date.
"Some parents are choosing co-education because they have not kept up to date with quite how fabulous some of the single-sex boarding schools have become," she said. "Once they come through our doors they are usually amazed and thrilled by what they see."
Margaret Rudland, headteacher of Godolphin and Latymer school in west London, said: "Parents are entitled to a choice and boys' schools that go co-educational are often better resourced, so the facilities look superficially better.
"I am concerned though because I want young women to get the best education, and when parents are shopping around they tend to look for facilities. If they dig deeper they will see we offer a first-rate education."
Elspeth Insch, head of King Edward's Handsworth state girls' school in Birmingham and committee member of the Association of Maintained Girls'
Schools, said: "There is a general feeling that girls' maintained schools are so successful that leaving us would be academic suicide.
"We lose a few at Year 11 to co-educational schools because they want to do soft option A-levels rather than the jolly solid academic ones we offer."