New credit card rules that took effect Sunday limit banks’ ability to charge penalty fees. (See our Best Credit Card Guide for tips on choosing and paying of credit cards) They come on top of rule changes earlier this year restricting issuers’ ability to adjust rates on the fly. Issuers responded by pushing card rates to their highest level in nine years.

In the second quarter, the average interest rate on existing cards reached 14.7%, up from 13.1% a year earlier, according to research firm Synovate, a unit of Aegis Group PLC. That was the highest level since 2001.

Those figures look especially stark when measuring the gap between the prime rate—the benchmark against which card rates are set—and average credit-card rates. The current difference of 11.45 percentage points is the largest in at least 22 years, Synovate estimates.

By comparison, the spread between 10-year Treasurys and a standard 30-year fixed-rate mortgage is just 1.93 percentage points, near historical averages, according to mortgage-data provider HSH Associates.

The moves are driven by a combination of forces. The Credit Card Accountability Responsibility and Disclosure Act of 2009 has given card issuers less flexibility to raise interest rates as they wish. At the same time, issuers are still dealing with credit-card delinquencies that remain above historical levels.

“The rules have changed and the goalposts of risk have changed,” says Paul Galant, chief executive of Citigroup Inc.’s (NYSE: C – News) Citi Cards unit.

[See a Guide to the Latest Credit Card Tricks]

Banks used to boost rates in a hurry on borrowers who fell behind on payments or otherwise turned out to be surprisingly risky. However, under the Card Act, financial institutions must warn customers at least 45 days before making substantial changes to rates or fees. People can avoid future rate increases and pay off existing balances over time.

As a result, most changes affect only new credit-card purchases. New rules that took effect Sunday limit what banks can collect in penalty fees, too.

At Discover Financial Services, a diminished ability to boost rates is causing the Riverwoods, Ill., company to offer fewer interest-free balance transfers for new customers, says Discover President Roger Hochschild. Balance transfers have declined 60% from last year. A typical offer might include 0% interest on the transferred amount for a year, with customers paying a balance transfer fee.

Rising interest rates on many credit cards won’t necessarily lead to more profits for issuers. “The interest-rate increases are designed to improve and protect profitability,” says John Grund, a partner with First Annapolis Consulting Inc., but stubbornly high delinquencies and Card Act curbs will eat into those gains, at least in the short term.