When Ester Lyndley received an “invoice” for almost £2,041 from King David school, Manchester, one of the country’s top state schools, she was panic-stricken.

Lyndley, a single mum, sends her children — Chanel, 7, and Joe, 5 — to the Jewish voluntary-aided school, which was rated the third best state school in the GCSE league tables last year. She claims she has been pestered for money by the school, which asks parents to contribute £1,200 a year per child.

In one letter Joshua Rowe, chairman of the trustees for King David infant, junior and high schools, told parents: “We need everyone’s contribution. It cannot be right that others should be funding your child’s education.”

The row has highlighted growing concerns that many of Britain’s top state schools regularly demand money from parents towards an education supposed to be free.

“Parents who cannot pay these ‘fees’ are being pressurised,” says Lyndley, who feels that King David has forgotten that it is a state school. While most of their funding comes from government, voluntary aided schools pay up to 10% of building work costs and for studies — such as Hebrew — that fall outside the national curriculum.

Last week, in response to a complaint from Lyndley’s lawyer, the government moved swiftly, insisting that the school must make clear that parental donations are entirely voluntary. In a letter to King David the DfES said it was “inappropriate” for schools to issue invoices and make phone calls chasing payments. The school was given 14 days to change its procedures.

Rowe insists parents knew the contributions were voluntary and denies there was any pressure on them to pay. After acknowledging that two of the school’s standardised letters would now have to be amended he appeared unrepentant, commenting: “Many schools charge much more than us.”

Margaret Morrissey of the National Confederation of Parent Teacher Association says such attitudes are not uncommon and warns the trend towards relying on parental contributions could become the norm. “The King David case is only exceptional in the amount being asked for,” she says.

Although few publicise the fact, several top schools ask parents to contribute to their costs via monthly standing orders or annual covenants. A monthly donation of £50 amounts, over seven years of schooling, to an unofficial charge of around £5,000 per pupil.

Morrissey traces the trend to the controversial request in 1999 from the London Oratory, the selective state school to which Tony Blair sent his sons, for parents to make £35 monthly payments. “I think it is sad that Tony Blair did not stamp on such demands there and then,” she says.

At Queen Elizabeth School for Boys, in London, head Dr John Marincowitz says parents have voluntarily been giving by standing order and covenant for years, though he will not say how much.

“I don't know what goes on at other schools because they can be coy about funding but it’s nothing new here,” he said. “I think most of our parents give that way.”

At Henrietta Barnett, in London, one of the country’s highest ranking grammars, parents have been sent letters suggesting regular donations. Some agreed. Georgina (not her real name) has a daughter at the school. “I was asked to contribute . . . and set up a standing order for £45 a month,” she says.

On the website of another selective state school, Queen Mary’s high school in Walsall, parents are asked to consider contributing regularly to school funds “if they are able”. The school needs new facilities, including science labs.

Why are parents at top schools under particular pressure to cough up extra cash? A parent at one London school, who prefers to remain anonymous, says one reason is that, to stay at the top of league tables, top state schools compete for staff with private schools, where salaries can be 20% higher.

“When my child started, parents were asked for money to buy books but eventually it was for teachers,” she says.

For Nick Seaton, of the Campaign for Real Education, the trend exposes the “blurring” of the lines between “free” state and paid-for private education. The DfES says the average spent by secondary schools on each pupil is around £4,450, not massively short of the average £7,200 private school fee.

Seaton says it would be fairer if all parents got a voucher to spend on the school of their choice, whether state or private.

Lyndley, meanwhile, wants ministers to stop schools making financial demands of parents. Or else, she says, they may as well “call these contributions an education tax and be done with it.”