Annual fees increased by an average of 5.8 per cent this year to pound;9,777, though in Wales they grew by 6.9 per cent, the biggest rise in the UK.
Eighty-eight schools now charge more than pound;20,000, compared to none four years ago. They include Harrow, where fees are pound;22,350, and Eton (Pounds 22,380). Pupil numbers dropped by 3,250 to 504,141 this year, the first decline since 1994. However, half of all parents have said they would privately educate their child if they could afford it.
The number of pupils in boarding schools - accounting for almost all the most expensive places - fell by 1.2 per cent overall. The figures have been collated by the Independent Schools Council (ISC), which represents 80 per cent of privately-educated pupils.
The figures come as profit-making companies such as Global Educational Management Systems (Gems) plan to offer parents a no-frills education at a fraction of the usual cost. Gems wants 200 schools, with fees as low as Pounds 2,000 a year, within 10 years, although critics doubt its schools can survive on such a budget.
The ISC says the drop in numbers is not a concern and that private schools have retained their market share nationally, as the school population as a whole dropped by 1.2 per cent this year. It blamed other factors for the fall, including a government decision to double visa charges for non-EU pupils to pound;500, leading to a 10 per cent decline in children from overseas. Jonathan Shepherd, the ISC general secretary, said the British Council was more obsessed with "filling its coffers" than representing British education overseas.
He said pupils from the Far East were being wooed by private schools in Australia and the United States, keen to exploit UK visa charges. The ISC said numbers had also been hit when Labour axed the assisted places scheme in 1997, putting an end to state-sponsored pupils in private schools.
The ISC census also revealed that there are now more girls (218,156) than boys (217,730) in private schools for the first time. At the same time, the number of pupils at the 207 schools represented by the Girls' Schools Association increased from 106,093 to 106,913, despite a drop in the overall private-school population.
Kavina Shah, 16, a pupil at the all-girl Queenswood school, Hertfordshire, said: "Being in an all-female environment instils self-confidence that I perhaps would not have had in a mixed school. You are not embarrassed to express yourself."
The gender split coincides with a change in the way private schools are represented. Last month the Headmasters' and Headmistresses' Conference, which represents the top private boys' schools, voted to include girls-only schools for the first time. And the GSA, representing 207 girls' schools, wants a more formal merger with the HMC.