Independent schools are suffering an income squeeze with parents struggling to meet fees as they battle against rising mortgages, a tightening of credit and reduced bonuses.

The proportion of schools owed money rose 35 per cent at the end of the last academic year compared with the same time in 2006, according to a Birmingham education finance expert.

And with worsening economic conditions since then, the number of parents experiencing difficulties in meeting education bills is likely to get worse, claimed Henry Briggs, senior partner at Birmingham-based HW Chartered Accountants.

Mr Briggs, one of the country's leading experts in education finance, discovered the growing fees deficit following a random survey of 20 independent schools.

He warned it showed school debtors were the now "highest he had seen for a generation".

Mr Briggs said: "The survey was done for the school year ending July/August last year. It is showing a trend which is likely to be getting worse during the current year.

"At the end of the academic year, schools shouldn't have debtors. If they have some left at the end of the summer term, that indicates there are fee-paying parents in the independent sector who are struggling with their payments.

"In the same way that they are having mortgage arrears, they are having them with school fees."

Mr Briggs warned if things got worse some families might have to drop out and go to the state sector. He said a general lack of confidence in the economy and an increase in bankruptcies was having a flattening effect across all sectors.

For independent schools, he said, the current dampening could hardly have come at a worse time, following several years of high-profile increases in the amount they charge.

Parents paying full fees at Birmingham's topperforming King Edward's School forked out £5,835 a year in 2000. That has now risen to £8,550.

"Fees have gone up by 40 per cent in the last five years," said Mr Briggs. "About 70 per cent of the money independent schools collect goes on salary costs. In the last five years there have been some very big increases for teachers in terms of national insurance and pension contributions that have to be absorbed."

So far parents have taken the increased costs "on the chin", said Mr Briggs. He put their willingness to pay down to continued dissatisfaction with state education, despite unprecedented spending over the past decade. "Some of the investment in the state sector hasn't necessarily reflected a perception of increased performance, which is a pity," he said.

"There was a time two or three years ago when it looked like state education was going to seriously challenge the independent sector in terms of excellence. It hasn't quite happened yet, though some of the academies coming through might change that."

Mr Briggs said independent schools needed to take "pre-emptive measures" against non-payment such as drawing up agreed payment schedules when arrears have arisen.

Parents could also be encouraged to take out insurance against unemployment or sickness.

"The threat to a school's financial health may be more than short-term cash flow or pupils leaving school with fees still outstanding," he said. "The cost of the negative publicity of attempting to collect the debt is unwelcome."

Independent schools have recently been put under pressure to more clearly prove their public worth in order to continue enjoying tax benefits they gain through having charity status. One way of showing this is the increased use of bursaries for low-income families.

Sarah Evans, head teacher of Birmingham's King Edward VI High School for Girls, sister school to King Edward's School which has boosted its assisted places scheme, said: "It would break my heart if any girl had to leave because of financial circumstances.

"The ethos at our school has always been to have access as wide as possible. We are fortunate that we have funds to help."