Shoppers are being overcharged £100m a year as a result of inflated interest rates on retail store cards, the UK's competition watchdog has said.
There are about 14 million store card accounts in the UK
The Competition Commission said there was evidence that the store card market was uncompetitive, with interest rates far higher than they should be.
The watchdog said it wanted warnings on card statements to inform consumers of interest rates and late payment fees.
There are about 14 million store card accounts in the UK.
Following an investigation, the Competition Commission said it had provisionally concluded that the market for offering consumer credit through retail store cards was uncompetitive.
The body said retailers and lenders were protected from competitive pressures and there was little incentive to reduce annual percentage rates (APRs) on store cards, which currently averaged about 30%.
This compares unfavourably to credit cards which commonly charge between 15% and 20% and the Bank of England base rate, which is currently 4.5%.
These "adverse" effects on competition were unlikely to be resolved over the next two years, forcing the Commission to take action.
"Consumers' sensitivity to APR levels and other charges is low," said Christopher Clarke, the Commission's deputy chairman.
"This results in store cardholders who take up credit, and associated insurance, paying more than they would in a fully competitive market."
The Commission's investigation focused on the general functioning of the market and not the behaviour of individual companies.
It is now consulting on a number of potential remedies, including forcing providers to put health warnings about rates on store card statements and improving information on payment methods.
The Commission's proposals for statements to include more consumer information will bring the store card market in line with the wider credit industry.
The credit card industry, following a parliamentary Treasury Select Committee enquiry into UK debt, agreed to introduce similar warnings on statements last year.
A Competition Commission spokesman told BBC News that greater consumer information was the way to tackle the high APRs being charged by store card providers.
"People, it seems, are genuinely ignorant about the rates they are signing up to when they take out a store card...if we raise the level of consumer knowledge then competition will improve and APRs, which are too high, should fall," the spokesman said.
The Finance and Leasing Association (FLA), whose members include all the major store card providers, said that in recent times APRs on offer from store card providers had become more competitive.
"We are now seeing lower store card APRs, the move to store-branded credit cards and greater consumer transparency," Ashley Holmes, head of legal affairs at the FLA.