Parents are spending an extra £45m a year because of restrictions on school uniform suppliers, the Office of Fair Trading (OFT) has found.
School governors usually set dress codes
The investigation found 84% of schools required that at least one item was bought from specified retailers.
The OFT said this was a tax on parents, mostly benefiting the chosen retailers, and called on school governors to scrap exclusive agreements.
More than 2,000 schools responded to a survey about their uniforms policy.
The OFT review was prompted by complaints over high prices and poor quality, and claims from retailers that the market was closed.
Profit for schools
The uniforms trade is thought to be worth about £450m a year, with most state pupils having to wear them.
Governors usually set dress codes, while schools can appoint a manufacturer to produce clothes which are sold directly to parents.
They can also select which outside retailers to use.
The review found uniforms bought from specified retailers cost parents about 23% more than if they shopped around in other outlets and 150% more than if they shopped in a supermarket.
Some schools claimed the restrictions improved convenience for parents and ensured better quality, the OFT said.
Parents buying secondary school uniforms were short-changed by £32m every year, while primary school uniforms cost £13m more.
The OFT calculated that if schools let parents use supermarkets, uniforms would cost £27 less for secondary pupils and £9 less for primary pupils.
'Over the odds'
A spokeswoman for Asda said a skirt, shirt and blazer from the supermarket chain cost £12.90 - about half the price of one blazer from a school outlet.
"There are many people on a tight income, regardless of school, who want to be able to clothe their children in quality school uniforms but they are being forced to pay over the odds," she said.
Citizens Advice said the findings reflected the experiences of low-income families.
OFT chief executive John Fingleton, said: "This restriction on competition acts as a tax on parents, which mostly goes to the chosen retailers.
"We call on school governors to eliminate these exclusive agreements."
The OFT's findings will be passed to the Department for Education and Skills.
A DfES spokesman said schools were advised to make sure uniforms were affordable.
He said: "School uniforms are good for discipline and school ethos, giving pupils a real sense of identity with their school.
"Heads who turn round failing schools tell us that uniforms play an important part in their work to raise standards."
Gillian Windass, of the National Governors' Association, said: "Each school has to look at its own policy and consult with parents.
"There may be certain circumstances where schools say: 'We like our exclusive uniform and we don't want to change it'. But is that an affordable option?"