There are calls for more schools to allow children to wear cheaper "off the peg" uniform.
School uniforms are chosen by individual governing bodies
Some parents complain uniforms are too expensive and that their schools insist they buy them from particular shops or suppliers, bumping up the cost.
An online survey of 1,000 people by the Citizens Advice Bureau, found nine out of 10 said they had to buy uniforms from a particular supplier.
UK schools were given warnings on uniforms by the OFT earlier this year.
The Office of Fair Trading said uniforms should be reasonably-priced and readily available and that schools could lay themselves open to prosecution if they forced parents to buy over-priced uniform.
Schools should not enter into exclusive agreements with retailers, it said.
The OFT published the results of an investigation last year which suggested primary school parents could save £9 a year and secondary school parents £27 a year if they were able to buy uniform at supermarkets.
It said that the vast majority (82%) of schools in the UK had a uniform.
In some schools, uniform was optional. England and Northern Ireland were most likely to have compulsory school uniforms (84% and 81% respectively), while in Wales it was compulsory in only 64% of schools and in Scotland for 75% of pupils.
The OFT's report said 84% of schools with a uniform insisted that some particular items had to be bought from a designated supplier or the school itself.
Following the report, the government published new draft guidelines for schools in England, telling them to make sure uniforms were freely available and affordable.
The guidelines are out for consultation at the moment and are due to be formally published in the next few weeks.
Supermarkets and other big retailers have been staging a price war on uniforms.
Earlier this year, Asda said a child could be kitted out for primary school for £10.
The store said the amount would buy a polo shirt, vest, sweatshirt, underpants, socks, trousers or skirt and shoes for a child aged between three and six years old.
The UK school uniform market is reported to be worth £450m a year.
Kim Maynard, policy officer for the CAB, said they had been highlighting the problem of expensive uniform for some years.
She told BBC's Radio Five Live Breakfast programme: "Parents feel really frustrated by the situation. They might be worried their child will be disciplined or bullied if they don't have the right uniform.
"You may think that seeing adverts for cheap school uniforms in chain stores, that things have changed but they haven't.
"Our survey showed that 87% of people still had to buy items from a specialist supplier. These can be much more expensive."
A spokesman for the OFT said: "We wrote to school governors and head teachers across the UK at the beginning of the year.
"What we asked them to do was reconsider any exclusive arrangements around supplying school uniform.
"Any arrangement might lead to higher prices, and in principle, any arrangement might be subject to enforcement action by the OFT."
Children, schools and families minister Kevin Brennan has also said schools must ensure uniform is affordable and widely available.
He said: "I am a complete advocate of uniforms in both primary and secondary schools. They can prevent bullying and add to the overall ethos of a school.
"For these reasons schools should make sure uniforms are not expensive to buy."