The UK's top public schools such as Harrow and Eton are being investigated for allegedly operating a cartel to push school fees artificially higher.
The Office of Fair Trading (OFT) confirmed that they were launching a fee-fixing investigation, but declined to specify which schools were involved.
The probe comes as parents face an average increase of 10% in school fees this year, thought to be one of the biggest rises on record.
A team from the OFT will now question bursars and headmasters at the schools and can demand that they hand over internal documents.
The bursars are alleged to have held secret meetings in order to ensure that rival schools across the board followed suit with substantial rises in fees.
The launch of the official probe comes after complaints by parents and an investigation by the Sunday Times newspaper.
"We have reasonable grounds to suspect a breach of the Competition Act," the OFT said in a statement.
The Sunday Times reports that the schools facing investigation include Eton, Harrow, Marlborough, Westminster and Bryanstson.
The maximum penalty for price-fixing is a five years' prison sentence, and the maximum fine is paying 10% of turnover for three years.