Parents are spending tens of millions of pounds more than they need to on school uniforms because of exclusive deals between schools and suppliers, the Office of Fair Trading said today.
Almost three-quarters of state primary and secondary schools stipulate a single outfitter for at least one item of uniform.
But OFT research showed that those retailers were charging up to treble the price elsewhere for skirts and trousers, costing parents an extra £5-£10 per garment.
As a result, it is writing to all 29,500 state schools in the UK asking them to review their uniform policies.
The watchdog first checked school uniform policies in 2006 following complaints from parents that they were being ripped off by a lack of competition.
Then, 84 per cent of schools were insisting at least one items being bought from a specific retailer, since when the Department for Education has recommended a relaxation of the policy.
In 2012, the OFT survey of 1,636 schools found that 74 per cent were still specifying a single supplier, for various reasons including quality and consistency.
On average, the OFT discovered each school was receiving £676 a year from the tie-up.
A mystery shopping exercise showed that a secondary school sweatshirt at a school-specified supplier was £12 - compared with £8 at a high street shop and £5 at a supermarket.
A secondary school girls' skirt could be a third of the price: £5 at a supermarket, against £15.40 at an appointed outfitter.
The OFT said that by giving greater detail on the required colour and style, schools could have "smart" school uniform policies that allowed parents to buy from anywhere.
Susan Oxley, OFT assistant director, said: "When schools require that uniforms are bought from a preferred supplier or shop it can act as a tax on families, which mostly goes to the chosen retailers.
"However, when families are able to shop around for school uniform items it can drive competition and bring down prices for all.
"We know schools don't want families to be left out of pocket and we have written to schools across the UK asking them to review their policies and supplier arrangements."