Jeremy Hunt is under pressure to axe the "school uniform tax" to help cash-strapped families.
A group of charities is leading calls for the Chancellor to scrap VAT on all school-specific items to help hard-pressed Brits in next week's Budget.
The full standard VAT rate of 20% is paid on clothing and shoes for children aged 14 and over.
But parents of younger children have been charged a zero rate for the same items since the introduction of the tax in April 1973.
Campaigners argued that charging VAT “unfairly penalises” older and taller children, while collectively costing families millions of pounds each year.
An open letter, backed by six charities including End Child Poverty and The Children’s Society, has now been sent to the Chancellor.
"School uniforms are vital to the fabric of our schools," the group said.
"However, we have become increasingly concerned that families across the UK are facing unprecedented financial pressures, and that this will be exacerbated when they have to buy new uniforms for their children during the school summer holidays."
The Schoolwear Association estimates removing VAT on school uniform across the whole UK would cost the Treasury £13million a year.
It claims the average cost of compulsory school uniform for secondary school pupils in England is £101.19.
New guidance was published in 2021 under the Education (Guidance About Costs of School Uniform) Bill to try and keep costs down.
For example, it states branding and logos should be kept to a minimum and parents must be allowed to buy cheaper items from supermarkets rather than expensive suppliers.
But campaigners have argued more needs to be done to protect parents in the cost of living crisis.
"Guidance on school uniform costs is welcome, but the Government can do more to help reduce the financial impact on hard pressed families," the group said.
"This is particularly important during the cost of living crisis, when families are already facing financial pressures and when we worry that inflation could increase uniform prices in summer 2023."
Liberal Democrat education spokesperson Munira Wilson backed the calls, telling the Mirror: "The hidden costs that come with sending a child to school or nursery are a struggle for thousands of parents.
"Yet the extra funding pots that schools have to support families with the cost of uniforms or school trips are often the first to be cut when their budgets are being squeezed.
"Schools are meant to keep expensive branded items to a minimum but ministers are doing next to nothing to check that these rules are being followed."
She warned that parents would judge the Tories for their "terrible record" on tackling the cost of living crisis.
The Labour MP Mike Amesbury added: “I was proud to change the law to help reduce the cost of school uniforms.
"This journey to assist hard pressed families and carers continues, I'm pleased to add my name to this coalition."
The Treasury was contacted for comment.
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