Girls at the top private schools produced 46 per cent of A grades achieved by all girls in the UK in last year's physics A-level, despite representing just 18.8 per cent of the national cohort.
The picture was similar in maths, chemistry, French and German, the analysis of results among the 207 members of the Girls' Schools Association shows.
The figures were cited yesterday by Clarissa Farr, president of the association, as she called for single-sex schools to remain.
Increasing numbers of girls' schools, including two run by the Girls' Day Schools Trust, a charity, have indicated that they will open mixed-sixth forms.
Ms Farr, head of the pound;20,895-a-year Queenswood boarding school in Hatfield, told the GSA annual conference in London: "The outstanding academic performance of girls in girls' schools is already well documented.
"But in the UK this has been given new force and significance against the prevailing background of national decline in the take-up of particular subjects, notably the hard sciences and modern foreign languages."
The GSA represents 207 leading girls' schools in the UK. Its conference has been staged jointly this year with the American-based National Coalition of Girls' Schools.
Ms Farr, who in 2006 will take over as high mistress of pound;11,844-a-year St Paul's girls' school, London, also called for responsibility for education policy to be taken out of the Government's hands.
She said a cross-party commission, similar to those established for national security and defence, should be established.