Results from recent GCSE exams show girls in single-sex schools consistently do better than those in mixed secondary schools.
Analysis published in The Guardian found girls were far more likely to thrive, get GCSEs and stay in education if they went to a single-sex school.
And pupils who had struggled benefited most from being in girls-only schools.
The research, into the GCSE scores of more than 700,000 girls taught in the state sector, was conducted on behalf of the Good Schools Guide. It found that, on average, all of the 71,286 girls who sat GCSEs in single-sex schools between 2005 and 2007 did better than predicted on the basis of their end-of-primary Sats results.
But of the 647,942 girls who took exams in mixed-sex schools, 20% did worse than expected.
Janette Wallis, editor of the Good Schools Guide, said: "A lot of parents will look at the benefits of co-ed schools, like the fact that girls and boys are educated side-by-side preparing them for the world of work and life.
"But to disregard this evidence would be a mistake. We never expected to see such a difference."
