If you carry a balance on your credit cards you could be in for a jolt.

Citibank, one of the nation's largest card issuers, is boosting interest rates for certain customers as it tries to cope with growing losses in its credit card portfolio. Some analysts think it won't be long before other companies follow suit to balance a sharp rise in defaults on credit card bills this year.

"A number of other issuers are in the same boat as Citigroup with the same market conditions and large losses," said Bill Hardekopf, chief executive officer at www.LowCards.com. "I wouldn't be surprised to see some other issuers making the same rate changes."

Late last week, Citi said it was raising rates by 2 to 3 percentage points for customers who have not had a rate increase in at least two years. The company wouldn't say how many customers would be affected by the new pricing, but published reports put the figure at about 20 percent of Citi's active U.S. accounts, or about 10 million card holders.

The increases come in the face of a string of interest rate cuts by the Federal Reserve aimed at spurring lending and bolstering the ailing economy. While the 1 percent federal funds rate is hovering near a 50-year low, the average rate on credit cards is roughly 14 percent.

Mr. Hardekopf said any rate increases on credit cards could backfire if the higher rates throw more cardholders into default.

Citi, which this week announced it was slashing more than 50,000 jobs on top of earlier cuts of about 20,000, issued a statement saying it needed to raise credit card rates because of "significant consumer credit deterioration driven by the mortgage crisis and rising unemployment."

Notices about the rate increases were included in monthly bills beginning this month. Customers who bank online and don't receive paper statements are being notified through a special mailing.

Customers can decline the increase by notifying the card company in writing. Those customers who opt out can continue to use their cards until the expiration date, but after that they must reapply for the card or find another issuer.

Customers who opt out also can continue to pay down their balances at the old rate, even after the card expires. Cardholders who don't opt out by January will be stuck with the higher rate.

Citi and other card holders should pay close attention to credit card mailings to watch for any notices about a rate increase, Mr. Hardekopf said.

The notices "often come in nondescript white envelopes that are easy to miss and toss before reading," he said.

He also advises cardholders to send any opt-out letters by certified mail, and to make a copy for their records.