Typically, the interest on credit card debt comprises borrowing cost determined by the U.S. Federal Reserve and an additional rate charged by the lender.

The additional rates, called the annual percentage rate (APR) margin, were raising costs for consumers by billions of dollars a year, CFPB said.

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Major credit issuers have increased the average APR margin by 4.3 percentage points over the last 10 years, the consumer watchdog said, adding it had found evidence of practices inhibiting customers' ability to find alternatives to expensive products.

The credit card market, being a highly-concentrated industry, has attracted scrutiny from regulators and lawmakers for years.

Concerns about competition were renewed this week after Capital One Financial agreed to buy Discover Financial for $35.3 billion, with analysts predicting tough antitrust scrutiny for the proposed merger.

CFPB said the APR margin for revolving accounts - a type of credit account that lets customers borrow up to their maximum credit limit - is now at 14.3%, the highest in recent history.

This has boosted the profitability of credit card companies.

Last week, the watchdog said that large credit card companies that dominate the market charge higher interest rates than smaller banks and credit unions.