Competition law has changed in recent years

It is investigating claims that schools exchanged information on costs and fees, in breach of competition law.

However, the OFT said the request for information was not evidence of an "infringement" by those approached.

The number of schools involved in the inquiry is thought to be more than 50.

'Unaware of change to law'

The Independent Schools Council said it had "serious concerns about the protracted nature of this investigation and the effect it may have on schools".

It appeared to acknowledge that some schools may have fallen foul of a change in the law in 1998 - but blamed the government for failing to keep them informed.

An OFT spokesman said schools had been served notices requesting disclosure of "relevant information".

ISC chairwoman Jean Scott wrote to OFT director general John Vickers 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."

Change

Mrs Scott said that under the legislation the 1988 Competition Act replaced - the 1976 Restrictive Trade Practices Act - private schools had been exempt from the anti-cartel rules that applied to businesses.

She said: "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."