American Express raises interest rates on portion of credit cards; average hike of 2.5 percent

American Express is increasing the interest rate on some of its credit card accounts by an average of 2.5 percentage points.

A spokeswoman for the company declined to say how many of its roughly 42 million cards would be affected. But she said Wednesday that it was a "small percentage" of its customers.

Spokeswoman Elizabeth Crosta said AmEx had been charging a lower interest rate of as much as 3.25 percentage points on its credit cards for customers with similar credit as competitors. She says the interest rate increase was authorized after this analysis.

Customers affected will be notified and will have the option to keep the card at the higher interest rate or cancel their cards.

The new interest rate only affects new purchases and balance transfers.