Some 17 banks and building societies have been forced to repay £149m of interest and charges to customers who took out credit deals.
The move comes after the Office of Fair Trading ruled that the financial institutions didn't give customers enough legal information about their credit agreements.
The repayments affect some 497,000 customers who took out store cards, credit cards, loans and hire purchase agreements. Under the Consumer Credit Act, a lender cannot recover interest or default charges for any period during which it has failed to comply with statutory requirements.
Customers should be contacted directly by their bank or building society if their agreement was affected. But Richard Lloyd from Which? said: "We'd like some assurances about how lenders will contact people who were affected but are no longer their customers."