Mr McIntosh sent a letter to parents - including Tony and Cherie Blair - on Wednesday, setting the levy at pounds 30 for one child and pounds 45 for two.
The missive read: "It will be impossible for us to maintain the current pupil-teacher ratio, continue to recruit and retain teachers of the highest calibre, reinstate a much-needed maintenance programme, restore departmental allowances to match the needs of the curriculum and generally to maintain high standards unless we raise funds to meet the shortfall in income from public funds."
The appeal, on the eve of the Labour Party conference, will be highly embarrassing for the Government, which is facing calls for the Chancellor's pounds 10bn "war chest" to be diverted to health and education rather than tax cuts. David Hart, general secretary of the National Association of Head Teachers, of which Mr McIntosh is a member, said: "I just find it very difficult to see how parents in a state school can be expected to pay. This is quite different from making a once-and-for-all payment for the purchase of books or equipment. I have a great deal of sympathy for the head and the governors of the school; they are in a very difficult position. I certainly hope it does not become more common because it raises serious questions about the funding of state education."
John Dunford, leader of the Secondary Heads Association, said the Oratory's proposal was unheard of. He added: "Schools used to spend parents' donations on extras. The expectations of schools are now so high that what was an extra is now core funding."
Requests for donations are banned from the new home-school agreements all parents must sign from this year. But there are no limits on headteachers' requests for help later in the school year.
Sir Bob Salisbury, head of Garibaldi School in Nottingham, who was knighted for his sevices to education, said: "Grant-maintained schools have been feather-bedded and were funded very generously indeed. It's a luxury if your parents can stump up anything and we would never try anything like that in this place."
Margaret Morrissey, of the National Confederation of Parent Teacher Associations, said parental giving already ran into tens of millions of pounds each year and called on the Government to issue guidelines to head teachers to limit requests for donations.
Estelle Morris, the School Standards minister, defended the Government's schools' funding policy. "I'm not commenting on the Oratory, I am explaining the financing of foundation schools.
"But let's be clear that what they are asking for, as many schools often do, is a voluntary contribution from parents. That's a matter for the head and the governing body. What we need to look at is the allegations that we do not have a fair funding system. We have done so much in the last two years to improve that."
Mr McIntosh was not available for comment yesterday.