The cost of sending a child to private schools has risen by about a fifth in the the last four years - around four times faster than rises in earnings, according to research.
It suggests that it is becoming increasingly difficult for some parents to afford to educate their children privately, with average fees now taking up more than a third of the average full-time salary.
The research, by Lloyds Bank is based on an analysis of figures on earnings and available data on school fees.
Average fees for a pupil attending a private day school now stand at around £12,345, it claims, up 21% from £10,176 in 2009.
At the same time, earnings for a full-time employee have risen by around five per cent, Lloyds concluded.
It goes on to say that private school fees have increased as a proportion of estimated earnings, suggesting that the current average school cost of £12,345 is equivalent to 37% of annual average gross full-time earnings of around £33,693.
"With school fees rising more than earnings, it is becoming more difficult for some to send their children to private school," Lloyds said.
The bank's analysis found variations around the country, with the highest fees seen in Greater London (£14,544 a year on average) and the South East (£13,920), while at the other end of the scale, average fees are £9,984 in the North and £10,431 in Scotland.
Just over a third of private school pupils receive financial help with the cost of their education, Lloyds concluded, with the average value of this support standing at nearly £4,687.
Sarah Deaves, private banking director at Lloyds Bank, said: "Private school fees have increased to almost £12,500 a year which is four times more than the rise in average earnings over the past five years. It is, therefore, becoming increasingly vital that parents plan ahead as early as possible, to ensure that they secure the future they desire for their children."
Matthew Burgess, general secretary of the Independent Schools Council, said: "Independent schools receive no state support for the world-class education they provide. The vast majority are not-for-profit educational charities and must look to fee-paying parents as their sole source of funding.
"There is no complacency on the part of school leaders who have a responsibility to safeguard the financial viability of the schools they run and are fully aware of the constraints on parents, who have already contributed through their taxes towards the cost of state schools. It's no surprise that parents will compare fee increases to inflation or changes in take-home pay, but the cost basis of a school is skewed towards teacher's salaries, pensions and national insurance contributions, utilities, food and maintenance of school buildings.
"Independent schools also increasingly subsidise the cost of their service for low-waged parents: bursaries funded by the sector now provide the same number of places as under the former assisted places scheme and are valued at more than £660 million annually."