The list of those involved reads like a "Who's Who" of the country's oldest and most famous public schools - including Eton, Harrow, Westminster and Winchester. The OFT has refused to rule out further investigations into fee-fixing covering more schools.
The 50 already identified face fines totalling millions of pounds if the findings are confirmed after a period in which the schools are given time to make representations.
The findings, after a two-year investigation by the OFT - described by officials as one of the largest in its history - provoked uproar amongst the independent sector. Leaders of the Independent Schools Council (ISC), which represents all of the schools concerned, said they had not been consulted about a change in the law - which meant private schools were brought under the umbrella of a law forbidding "anti-competitive" collaboration over fee rises.
The OFT said "regular and systematic exchange of confidential information" between the 50 had led to parents being charged higher fees than they would have otherwise been. In the three years covered by the charges - 2001 and 2004 - school fee rises were often as much as three times the level of inflation.
Fines can be as much as 10 per cent of an organisation's income for every year it breaks the rules. In the case of the larger institutions such as Eton, that could mean as much as £7.5m. But officials from the OFT indicated the fines were likely to be set at a lower level.
The OFT has ruled that regular contact between the 50 - conducted through Sevenoaks School, which collated information about fee rises that was then passed on to the others - was contrary to the Competition Act of 1998.
The information was regularly updated and recirculated to the schools involved between four and six times each year as they went through their annual budget-setting processes.
The OFT said the ruling "only relates to this particular agreement". A spokesman added: "There could be more investigation into other agreements"
The ISC said the inquiry had been "a scandalous waste of public money". Jonathan Shephard, its general secretary, described the situation as Kafkaesque. "The law seems to have changed without Parliament realising - and without the independent sector being consulted - contrary to the Government's own strict guidelines on consultation," he said.
As a result, independent schools may issue a direct legal challenge to the OFT's ruling. "Schools are now being held liable for breaking a law which no one knew applied to them," Mr Shephard added.
He rejected the claim the collaboration had led to higher fees. "Schools keep their fees as low as practicable," he said. "Sharing information is an effective - though no longer legal - way of doing this."
The 50 guilty schools
* Ampleforth College, York, £20, 520 a year
* Bedford School, £19,812
* Benenden School, Kent, £22,350
* Bradfield College, Berkshire, £21, 750
* Bromsgrove School, Worcestershire, £18,990
* Bryanston School, Dorset, £21,807
* Canford School, Dorset, £22,080
* Charterhouse, Surrey, £22, 184
* Cheltenham College, Gloucestershire, £21,450
* Cheltenham Ladies College, Gloucestershire, £21,729
* Clifton College, Bristol, £20, 670
* Cranleigh School, Surrey £22,350
* Dauntsey's School, Wiltshire, £19,035
* Downe House School, Berkshire, £22,875
* Eastbourne College, Sussex, £20,085
* Epsom College, Surrey, £22,389
* Eton College, £23,588
* Gresham's School, Norfolk, £20,670
* Haileybury School, Hertfordshire, £20,955
* Harrow School, £23,100
* King's School, Canterbury, £23,280
* Lancing College, Sussex, £21,885
* Malvern College, Worcestershire, £21,990
* Marlborough College, Wiltshire, £21,900
* Millfield School, Somerset, not available
* Mill Hill School, London, £19,740
* Oakham School, Rutland, £18,330
* Oundle School, Peterborough, £20,691
* Radley College, Oxfordshire, £21,360
* Repton School, Derby, £20,025
* Royal Hospital School, Ipswich, £16,908
* Rugby School, Warwickshire, £21,750
* St Edward's School, Oxford, £21,624
* St Leonards-Mayfield School, Sussex, £19,800
* Sedbergh School Cumbria, £20, 700
* Sevenoaks School, Kent, £20, 199
* Sherborne School, Dorset, £21,675
* Shrewsbury School, £22,590
* Stowe School, Buckinghamshire £22,980
* Strathallan School, Perth, £19,512
* Tonbridge School, Kent, £21,122
* Truro School, Cornwall, £16,479
* Uppingham School, Rutland, £22,500
* Wellington College, Somerset, £14, 401
* Wells Cathedral School, Somerset, £19,350
* Westminster School, London, £21,948
* Winchester School, Hampshire, £23,500
* Woldingham School, Surrey, £21,240
* Worth School, Sussex, £21,165
* Wycombe Abbey, Buckinghamshire, £23,100
Source: 2005-06 Independent Schools Yearbook; maximum boarding fee.
Finding of guilt covers three years from 2001-02 to 2003-04 except in the cases of Truro and Sedbergh, which only covers the first two years.Reuse content