Schools in England that unfairly profit from uniform sales face the threat of prosecution under a new crackdown.
School governors usually set dress codes
The Department for Education and Skills says the Office of Fair Trading could act against exclusive deals with outlets to sell over-priced uniforms.
New guidance will tell schools to ensure their uniforms are affordable and sold in a range of shops.
Expensive uniforms can discourage less affluent families from applying to certain schools, ministers say.
A recent OFT survey found parents were overcharged for uniform by £45m a year.
The survey found bills were one-and-a-half times higher when uniforms were sold through exclusive outlets rather than supermarkets.
And exclusive deals proved nearly a quarter more expensive than uniforms sold through school retailers generally.
Ministers have been concerned that expensive uniforms can be used by schools as a back-door selection process - deterring less affluent families from applying to a certain school.
The new DfES guidance tells all schools in England to limit the cost of uniforms and make sure they are widely available off the peg from High Street shops, other retail outlets and internet suppliers.
Schools Minister Jim Knight said: "Wealth should never be a barrier to opportunity.
"I want every child to be able to go to the school of their choice and unfair and expensive uniform policies can prevent this.
"As our new guidance makes clear, schools should look to set affordable uniform policies that do not seek to make additional money through expensive and exclusive contracts."
He added that as schools had received record levels of funding there was no need to profit from uniform sales.
He said the amount passed on to the school could be minimal in comparison to its impact on parents.
"In extreme circumstances, schools could face prosecution by the Office of Fair Trading," Mr Knight said.
A letter from OFT chief executive John Fingleton sent to all the chair of governors in England said: "A school's uniform policy is the responsibility of its board of governors.
"I urge you as the chair of governors to ensure that this matter is considered by your school's governors in the near future.
"Steps should be taken to end any exclusive contracts and for your school's uniform to incorporate items that are readily available off the peg from a number of retail outlets.
"This will enable parents to shop around for the most cost-effective options and ensure that uniforms are competitively priced."
The updated guidance is due out in the spring.