The government has issued guidelines on the price of school uniforms
The cost of sending children to school is putting many parents under pressure, two charities are warning.
A YouGov poll of 2,128 adults for Barnardo's and Citizens Advice found 73% of parents said the expense of uniforms was a source of stress.
Government guidelines issued to schools in England say school uniforms must be reasonably priced and widely available.
But Citizen's Advice said many schools were ignoring the advice, aimed at helping parents on low incomes.
It said seven out of the eight schools in one English borough were not following the advice, issued by the government last year in a bid to help parents on low incomes.
Delroy Pomell, director for education at Barnardo's, said: "Children and families living in poverty face numerous challenges including poor housing, poor health and poor nutrition.
"Starting the first day of school with the right uniform shouldn't become yet another source of stress."
Adrian Galvin, campaigns officer at Citizens Advice, said: "Citizens Advice Bureaux regularly report seeing clients who are struggling to meet the escalating costs of schooling their children.
"The cost of uniform, PE kit, school trips and other specialist equipment can run into hundreds of pounds per child each year.
"While some schools are following the school admissions code - government guidance regarding the fair pricing of uniform and other school costs - many schools are continuing to ignore it.
"We are urging the government to strengthen the code and the way it is regulated to compel schools to follow the guidelines."
Kevin Brennan, Minister for Young People and Families, said: "Our guidance is clear that the cost of uniforms must not exclude or deter families from poorer backgrounds."
The government says parents who are unhappy about the cost of their child's uniform should raise their concerns with the school.
YouGov surveyed 2,128 adults between 27 and 29 August, 2008.