Schools in England will be told to take action to cut the cost of school uniforms to help hard-pressed parents.
Education minister David Laws is to issue new guidance to end the practice of using a single uniform supplier, enabling parents to shop around.
He told BBC Breakfast he wanted to stop profit-sharing schemes that existed between some schools and shops.
But the biggest teaching union said schools could merely ignore the "weak" orders, adding legislation was needed.
The Local Government Association has blamed soaring uniform costs on schools rebranding as academies with new logos.
Mr Laws, who will make the announcement at the Lib Dem conference in Glasgow, said the cost of uniforms was often "unnecessarily high" at a time when family budgets were being squeezed.
'Profit from the business'
He said: "The fact for example that it is too easy for schools to have these single supplier arrangements where parents can end up paying more than they need to.
"And there are also things that we feel we should get rid of.
"For example, it's possible at the moment to have cash back arrangements where schools can have these arrangements with single suppliers and in return get some of the profit from the business."
School uniforms, he added, could be an important sign of identity and pride but, at a time when many family budgets were squeezed, parents should not be forced to spend more than they needed to.
Mr Laws said: "We will send a strong signal to schools that it is vital to secure value for money for parents before changing or introducing new school uniforms. Parents need to be able to shop around to find the best deal."
He denied new laws were needed and was confident schools would follow government guidance.
Schools in England will be urged to stop using exclusive single supplier contracts, unless regular tendering processes are run to ensure firms provide value for money.
The guidelines will also say schools should not enter into 'cash-back' agreements with shops and compulsory items of uniform should be available relatively cheaply.
Branding should also be kept "to a minimum", the guidelines will say, and schools will be urged to avoid changing specifications frequently.
An Office of Fair Trading (OFT) investigation last year suggested 75% of UK state schools placed restrictions on where uniforms could be bought.
Such a move typically added £5 to the price tag for each item, leaving parents an estimated £52m a year worse off, the OFT said.
The announcement follows a Labour Party campaign to highlight the cost of school uniforms.
David Cameron has twice responded to calls for action on school uniform prices by Labour MPs at prime minister's questions, accusing them of wanting to undermine the government's flagship free schools programme.
He told the MPs schools should be free to set their own "tough and robust" uniform policies.
Chris Keates, general secretary of the NASUWT - the largest teachers' union - said the coalition had scrapped rules forcing schools to restrict costs.
"Parents are now paying the price of the excessive freedoms this coalition has given to schools.
"This announcement largely repeats the guidance the Office of Fair Trading gave in a letter it sent to all schools last year, which many of them ignored.
"If the Liberal Democrats are serious about this issue they need to commit to issuing statutory guidance rather than weak advice."
But Russell Hobby, general secretary of National Association of Head Teachers, said: "Many schools go to great lengths to make their uniforms both smart and affordable; helping out where they can those families who struggle to afford them."
Last month, the Local Government Association said schools had a "moral duty" to keep costs down for parents.
It said schools which decided to alter their uniform - for example a newly-converted academy that decides to change its emblem - should restrict changes to one or two items or to sew-on logos.