WASHINGTON  — Federal regulators today adopted sweeping new rules for the credit card industry that will shield consumers from increases in interest rates on existing account balances among other changes.

The rules, which take effect in July 2010, will allow credit card companies to raise interest rates only on new credit cards and future purchases or advances, rather than on current balances.

They were approved this morning by the Office of Thrift Supervision, a Treasury Department division. The Federal Reserve and the National Credit Union Administration were expected to act on them later in the day. The changes mark the most sweeping clampdown on the credit card industry in decades and are aimed at protecting consumers from arbitrary hikes in interest rates or inadequate time provided to pay the bills.

John Reich, the thrift agency’s director, said the rules “will enhance public confidence in financial institutions and establish a level playing field for institutions that want to do business fairly without suffering competitive disadvantages.”

Most of the rules were first proposed in May and drew more than 65,000 public comments — the highest number ever received by the Fed. They also restrict such lender practices as allocating all payments to balances with lower interest rates when a borrower has balances with different rates.

But the changes also could make it more difficult for millions of people with bad credit to get what is known as a subprime card carrying higher interest rates, some experts say.

In addition, consumers will have to be given 45 days notice before any changes are made to the terms of an account, including slapping on a higher penalty rate for missing payments or paying bills late. Under current rules, companies in most cases give 15 days notice before making certain changes to the terms of an account.

The changes could cost the banking industry more than $10 billion a year in interest payments, according to a study by the law firm Morrison & Foerster.

Roughly 16,000 companies in the U.S. issue credit cards. The biggest lenders include Discover Financial Services LLC, Bank of America Corp., Citigroup Inc., JPMorgan Chase & Co., Capital One Financial Corp., American Express Co. and HSBC Holdings.

The head of the American Bankers Association called the changes “strong new regulations ... (that are) unprecedented in their scope and signal the beginning of a new market structure for credit cards.”

“While the new rules are designed to increase protections for consumers, the Fed itself has recognized that they may result in increased costs for most card users and reduced credit availability, particularly for consumers with lower credit scores or limited credit history,” ABA President and Chief Executive Edward Yingling said in a statement. “With the uncertainty facing our financial system, it’s absolutely vital for policymakers to understand the full impact of these regulations on consumers and the economy before judging their success or further restricting the marketplace.”

The new rules prohibit:

n Placing unfair time constraints on payments. A payment could not be deemed late unless the borrower is given a reasonable period of time, such as 21 days, to pay.

n Placing too-high fees for exceeding the credit limit solely because of a hold placed on the account.

n Unfairly computing balances in a computing tactic known as double-cycle billing.

n Unfairly adding security deposits and fees for issuing credit or making it available.

n Making deceptive offers of credit.

Under the new rules, credit card lenders will be required to apply any payment above the minimum to the part of the balance with the highest interest rate.

The so-called subprime cards for people with low credit scores typically have no more than a $500 credit limit but require a large upfront fee.

The rules cap that fee at 50 percent of the credit limit and allow the cardholder to pay off the initial balance over a year, not immediately.

The Consumer Federation estimates that credit card debt held by U.S. consumers is about $850 billion, some four times what it was in 1990.

Associated Press writer Carson Walker in Sioux Falls, S.D., contributed to this report.