US Edition At least 60 private schools have been ordered to hand over documentation on their finances to an inquiry into alleged fee-fixing.
The Office of Fair Trading has been investigating claims for some months that top independent schools regularly exchanged information on their costs and likely fee increases in breach of the 1998 Competition Act.
The OFT's move provoked protests from the Independent Schools Council, which said it had "serious concerns about the protracted nature of this investigation and the effect it may have on schools".
However, the ISC appeared to acknowledge that some schools may have fallen foul of a change in the law, but blamed the Government for failing to keep them informed.
An OFT spokesman said that a "significant" number of schools have been served notices requesting disclosure of "relevant information", which she stressed was not evidence of any "infringement".
Jean Scott, the ISC chairman, wrote to John Vickers, the OFT director-general, saying it was unclear why private schools were being "subjected to treatment that appears to be neither swift nor proportionate".
She added: "They are not a group of businessmen meeting behind closed doors to fix the price of their products to the disadvantage of the consumer.
"They are schools that have quite openly continued to follow a long-established practice because they were unaware that the law had changed."
Mrs Scott said that under the legislation the Competition Act replaced, the 1976 Restrictive Trade Practices Act, private schools had been exempt from the anti-cartel rules that applied to businesses.
She accused the Department for Trade and Industry of failing to "see fit to consult the sector" before the 1998 legislation removed this exemption.
"Many people, not unreasonably in our view, thought the Act was more relevant to oil companies, supermarkets and retail chains than a charity, let alone a school.
"There seems to have been a failure on the part of Government in getting this information across to the sector," she said.