Marks Spencer launched a credit service that allows customers to borrow up to 500 dollars in a matter of minutes.
The credit account Sparks Pay is reminiscent of buy-now pay-later deals and could lure financially stretched families into debt, critics warned.
The service is available for around 16 million members through the M&S Sparks loyalty scheme.
It will first be offered to customers shopping online and via its mobile app before being rolled out in the high street next year.
Shoppers will be asked to fill in an application form to qualify for an account. Within a few minutes a decision will be made.
If the balance is returned within 45 days or 76 if it is their first order, there will be no interest charged.
After that, balances incur interest at a rate of 23.9 per cent. It is not yet known how the service will work in physical stores.
Labour MP Stella Creasy said: I am not surprised that brands are doing this because there is so much to be made from pushing people to spend money they don't have.
M&S isn't doing this out of the kindness of their heart to help you budget, but they're doing it to help you keep spending at their shop. Consumer champion Martyn James said that instant credit encourages people to spend money they don't have and interest-free deals are easy to sleepwalk into.
Everyone thinks they will pay the money back in time but companies are betting on the fact that they won't. The high street giant denied its Sparks Pay credit account resembled a buy-now pay-later scheme because it is not regulated by the Financial Conduct Authority, unlike firms such as Klarna.
M&S checks customers' debts and payment histories to determine how much they can borrow.
M&S said that should any M&S Bank customer have financial difficulty and need additional assistance, we have a dedicated financial support team that would be able to support and discuss the right solution for the customer.