Parents are propping up state schools by contributing more than pound;200 million a year towards the basics, including books and salaries.
Charities and trusts are pumping hundreds of millions more into the system as schools struggle to make ends meet.
Margaret Morrissey, of the National Confederation of Parent-Teacher Associations, said: "The concept of a free state education is being eroded and parents no longer even question the amounts they are expected to hand over."
Fundraising groups said they were receiving unprecedented numbers of calls from schools asking for help to raise cash.
The disclosure follows a warning by MPs that the sense of crisis surrounding school funding would continue this year. They accused ministers of promising more than they could deliver.
School funding changes, implemented last year, led to more than 60 per cent of secondaries and 56 per cent of primaries reporting smaller budgets. An estimated 8,800 posts were lost as a result.
At the same time local authorities have attempted to cut costs by allowing schools to charge pupils, illegally, for singing lessons. A survey by The TES last month suggested that this is the practice in most authorities.
A growing number of councils have also stopped providing grants to help poor parents buy uniforms. Essex became the latest to abandon these on Wednesday. Plans to charge parents for children's bus trips to school will be debated in Parliament this year.
Ms Morrissey said parents were providing at least pound;60m for schools through fund-raising events. The rest came from donations with schools urging parents to covenant money. The London Oratory, attended by the Prime Minister's two elder sons, asked its parents for up to pound;35 a month.
A survey by American Express last year found that parents were paying pound;1,197 per child on extras, including trips and stationery.
Ms Morrissey said: "Three years ago it was common to receive calls from parents who were concerned that money raised by them was going on essentials. Now they just accept that their cash will go on books and even teachers' salaries."
Mark Jefferies, managing director of fundraising consultancy Craigmyle, said enquiries from state schools had increased five-fold in the past three years.
Mr Jefferies, who is also on the board of the Association of Fundraising Consultants, said many of these were from schools looking for ways to raise cash so they could apply for specialist status. But others needed money to supplement their budgets.
Several schools, including Ponteland county first school in Northumberland, said last year that parent fundraising was crucial to cover essential costs, such as staffing. Rachel Earnshaw, head of Soho Parish primary in London, said an art auction held by her PTA in May had been vital to her school's survival. Artists including Damien Hirst and Gavin Turk donated work and it raised pound;150,000 Sheila Lawlor, director of the social and economic policy think-tank, Politeia, said: "I do not have any qualms about people contributing to their children's education, as long as those who cannot afford it are not disadvantaged.
"Parental contributions force schools to be more accountable for the type of education they are providing and how money is spent."
Charles Clarke, Education Secretary, has said that schools' funding had increased by 25 per cent in real terms, per pupil, since Labour came to power in 1997. A spokeswoman said that spending per pupil was pound;2,810 in 19978, and will rise to pound;3,600 in 20034 and pound;3,700 in 20045.
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