The price of school uniforms has gone up by £10 an item where schools appoint exclusive uniform suppliers, the Competition and Markets Authority (CMA) said in an open letter. The letter said that school governors must guarantee that deals allow parents to get great value for money and that there is "competition between suppliers and retailers".
The CMA was prompted to send the communication to governors and suppliers in England following complaints from parents. It has the authority to fine suppliers if they are found to be anti-competitive.
CMA senior director Ann Pope said: "Buying school uniforms can be very expensive and particularly hits low income families and those with a number of children, so it is important parents and carers are able to shop around.
"We urge everyone involved to ensure that they are providing a good service to parents and carers and complying with Department for Education guidance. We will continue monitoring the sector and will consider taking enforcement action, if it is necessary."
The CMA published the open letter after work by the Office of Fair Trading in 2012 showed that 74% of state schools in England had limitations on where school uniform could be brought. These restrictions meant that parents were paying an extra £5 to £10 for individual items.
Sam Royston, director of policy at The Children's Society, said: "School uniform costs can be a millstone around the necks of poorer parents, contributing to a cycle of debt and damaging the opportunities and well-being of lower income pupils. One reason for the high costs are policies that force parents to buy clothing from specialist shops, and prevent them from buying cheaper items from supermarkets."