The number of children at independent schools in England has fallen slightly over the past three years, government figures show.
But demographic factors mean the 431,650 fee-paying pupils are a marginally higher proportion of the school population than they were in 2004.
The numbers of children born to middle class families tend to remain stable, while there are greater fluctuations among lower socioeconomic groups. This explained why university applications remained buoyant in the 1980s and early 1990s despite large falls in the number of 18-year-olds.
This stability among families able to afford private education means that independent secondary school numbers have remained almost constant, while there has been a fall of about 6,000 in primary.
Nevertheless, the increased proportion of privately education secondary pupils - up from 7.1% to 7.3% - was enough to trigger attacks from political opponents.
David Laws, the Liberal democrat education spokesman, said: "These rises clearly show that in some parts of the country parents are so concerned about the standards of local education they are willing to pay a lot of money to send their children to private schools.
"Ministers must be disappointed that this continuing rising share appears to be a vote of no confidence in their policies, particularly in areas such as London where state education is seen not to be delivering."
A spokesman for the Department for Children, Schools and Families said: "The overwhelming majority of parents are clearly satisfied with the state education system. New research from Keele University shows that nine out of 10 are happy with their children's schools.
"The state school system is continuing to deliver for parents, regardless of their wealth or background, with record results across the board," he added.
The figures showed striking variations across England.
In the north-east region, for example, only 3.8% of secondary school pupils were independently educated this year.
But in inner London, the figure was more than three times as high at 13.7%.
In the affluent London borough of Kensington and Chelsea, 44.2% of 11 to 15-year-olds were attending private schools in 2007. In Cornwall the figure was just 4%.