The number of pupils attending private school has risen by 5.6% since 2001
The cost of sending a child to private school in England has risen by 43% since 2003, according to a survey.
Research by Halifax Financial Services suggests that average annual fees now stand at £10,239 - up 6% on £9,627 a year ago.
The average in south-east England is 27% higher than in the North.
But the Independent Schools Council said private education was good value for money and many schools were much cheaper than the £10,239 average.
The number of pupils attending private school in England rose by 5.6% between 2001 and 2007. In the same period, the state sector saw a 3.4% drop in student numbers.
Architects and pharmacists
The survey claims that private education is now affordable for parents in just 18 professions, compared with 30 occupations in 2003.
Halifax classed as unaffordable fees which amounted to more than a quarter of the average gross earnings for a given job.
On that basis, it said, teachers, architects, police officers and pharmacists were now among those unable to afford private schooling for their children.
Halifax said the biggest increase in school fees was in the south-west of England where they have risen on average by 48%, from £7,188 in 2003 to £10,671 in 2008.
The slowest rise, of 30%, was in the West Midlands.
AVERAGE PRIVATE SCHOOL FEES IN ENGLAND
South East - £11,379
South West - £10,671
North - £8,361
Greater London - £11,361
South-east England remains the most expensive region, with average fees of £11,379, followed by Greater London and the South West.
Average fees in the North of England are £8,361.
Martin Ellis, chief economist at Halifax Financial Services, said fees had risen by significantly more than inflation.
The Independent Schools Council (ISC) criticised the survey's assumptions. It said, for example, that while trainee architects might not be able to afford private schooling, it was untrue to say that no-one in the profession could.
And it said the findings did not take into account the fact that many day and prep schools were much cheaper than Halifax's average.
That figure, the ISC said, was skewed by a small number of boarding schools and top-end public schools which charge very large sums.
Pru Jones, from the ISC, told the BBC that private education was still good value for money.
"What has to be remembered is you have put [fees] in the context of the cost pressures that schools are facing, and you have to put them in the context of the rises in other costs of education as well," she said.
These costs include an increase in staff wages and pension contributions, as well as higher utility bills and insurance costs, the ISC said.