After a two-year investigation the Office of Fair Trading is believed to be on the verge of finding dozens of schools, including Eton and Harrow, guilty of conspiring to set fee levels.
The decision, expected next month, could result in fines totalling 10 per cent of schools' turnover over the past three years - as much as pound;10 million for the top boarding schools, which all charge pupils more than pound;20,000 a year.
Jonathan Cook, secretary general of the Independent Schools' Bursars Association, said some schools had already set aside money to cover the fines but insisted they would appeal against any guilty verdict.
He said schools were not alerted to a law change contained in the 1998 Competition Act, which banned any exchange of information on fees, a widespread practice among private schools in the past.
"These changes to the Competition Act were made without any consultation with schools. No one knew about them, so how can they find us guilty?" he said.
The investigation, which centres on more than 60 schools, started in June 2003 after evidence emerged of meetings between bursars to discuss fee increases.
It was claimed parents were forced to pay high prices as there was little difference between the fees charged by certain types of schools.
Many schools involved in the investigation have never denied that they shared information about fees, but said they did this in good faith to ensure that parents were not overcharged.
The OFT, which is thought to have spent more than pound;200,000 on the investigation to date, denied it had already made its ruling, saying a verdict may not be released until next year.