Thousands of state schools will be made to review their uniform policies to allow parents to shop around for cheaper items, Office of Fair Trading has ruled.
The announcement comes a day after students protested outside Westminster against the astronomical cost of uniforms.
Yorkshire teenagers from the Save the Children 'Our Shout' group donned school uniforms and market trader outfits on Wendesday to emphasise the prohibitive cost of uniforms for poorer families.
In some areas of England, the cost of a school blazer is more than £90.
Current government guidance states "schools should ensure that the chosen uniform is affordable and does not act as a barrier to parents when choosing a school". Despite this, some schools are charging in excess of £100 for school uniform, in a time where school uniform grants are being slashed.
Now, The OFT has asked almost 30,000 state schools to review their uniform policies to allow families to choose where they buy uniforms from.
The OFT's request follows a survey which found that items could be as much as £5 to £10 higher when bought from retailers or suppliers selected by the school or from their own shops.
It said families could save tens of millions of pounds if primary and secondary schools removed restrictions and allowed them to shop around freely.
The study also found that 74% of state schools continue to place restrictions on where parents must buy some uniform items.
In one example, the average price of a secondary school's sweatshirt was £12 under a restricted arrangement compared with an indicative cost of £8 at competing retailers and £5 at a supermarket.
The survey found that schools use a single supplier for a number of reasons, including wanting a consistent, good quality uniform and convenience for parents.
The OFT said schools could still have "smart" school uniform policies without appointing a single supplier.
It suggested that schools could achieve consistency through setting out colour and style requirements in more detail but still allow parents choice about where to buy items.
Susan Oxley, assistant director in the OFT's goods and consumer group, 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."