Six Midland public schools face fines that could total millions of pounds after the competition watchdog found they were involved in an illegal fee-fixing cartel.
In a provisional ruling, the Office of Fair Trading criticised 50 of Britain's most prestigious public schools for sharing information on their plans for fee rises.
In one of its largest investigations ever, the OFT said their actions resulted in parents being charged more than they otherwise would have been.
The six Midland schools facing fines are Bromsgrove School, Cheltenham College, Cheltenham Ladies College, Malvern College, Rugby School and Shropshire's Shrewsbury School.
The schools have until March next year to respond.
Hugh Carson (pictured), head teacher of Malvern College, said: "Obviously we have to consider what they have said very carefully and work out our response."
Lady Fiona Mynors, President of Council at Cheltenham College, denied its parents were charged higher fees, adding the school would be replying to the OFT in "due course".
A spokeswoman for Cheltenham Ladies' College said: "We will now be considering our response. The Chairman of Council is today writing to all parents and we will keep them informed over the coming months." The Independent Schools Council-last night blasted the OFT's investigation as a "scandalous waste of public money".
It said private schools were being unfairly treated like businesses when, in fact, they were non-profit-making organisations with charitable status.
Jonathan Shephard, general secretary of the ISC, said: "OFT has failed to understand that charities have no motive for raising more money than is needed for charitable activities.
"All surpluses stay within the charity and must be used for charitable purposes. There is no possibility of any personal gain."
The ISC warned any fines would have to be passed on to parents which would mean "damaging the very people who the OFT is supposed to protect".