Next Christmas, when it comes time to pay off holiday shopping charged to your credit card, there's a good chance you'll be sending money to Bank of America's credit-card unit. And according to some consumer advocates, there's a good chance you'll be paying higher interest and fees.

Bank of America became the largest issuer of credit cards in the nation Jan. 1 when it completed a $35 billion acquisition of MBNA Corp., the third-largest U.S. credit-card bank.

The sheer size of the combined credit-card business, which will be based in Wilmington, Del., has consumer advocates fearing that Bank of America will be able to use its dominance to take advantage of cardholders. They argue that joining Bank of America and MBNA will reduce competition, leading to further escalation of fees and interest rates for consumers. Bank of America will have about 118 million cards in circulation and about $140 billion in cardholder balances.

"Consumers should expect to pay more on their credit-card bills because of this acquisition," said Rashmi Rangan, executive director of the Delaware Community Reinvestment Action Council, a consumer advocacy group focused on banking.

Based on past experience, Laura Atwell, a freelance television journalist, said she's concerned about Bank of America expanding its U.S credit-card business. The Wilmington woman said she and her husband canceled a Bank of America credit card a few months ago over what they considered unreasonable increases in the card's interest rate, which had climbed to about 30 percent annually because of a disputed late payment.

"I think these big banks have way too much power," Atwell said. "I think they should be regulated."

Other consumers are less worried. Andrew Guile, an environmental consultant who got into a dispute with MBNA last year over an increase in his interest rate, said he expects he'll still have plenty of choices even after Bank of America completes the buyout.

"You can find another credit-card company at the drop of a hat," said Guile, who lives in Milltown, Del., and remains an MBNA cardholder.

Now the fifth-largest U.S. credit-card issuer, Bank of America's market share will jump from 8.6 percent to 20.3 percent after it absorbs MBNA, according to statistics from the Nilson Report, a credit-card industry newsletter.

That market share will put it slightly ahead of the current No. 1 issuer, JPMorgan Chase & Co., another megabank with its credit-card unit based in Wilmington. Chase's market share is 19.1 percent.

The top 10 credit-card issuers collectively represent more than 85 percent of the U.S. market, according to the Nilson Report. And, once MBNA is absorbed, the top three alone — Bank of America, JPMorgan Chase and Citigroup — will control roughly 55 percent.

For its part, Bank of America downplays concerns that it's gaining too much market power. Although tight-lipped about certain specifics, such as whether MBNA cardholders can expect rate changes, Bank of America executives have told regulators the buyout will be good for consumers.

According to Bank of America, MBNA customers will benefit because they'll have access to Bank of America's comprehensive financial services, including checking and investment accounts.

MBNA never pursued a traditional banking strategy, instead focusing on lending money through credit cards. It was only in recent years that MBNA started to push into mortgages and other types of lending in the face of slowing growth in its core credit-card business.

Bank of America, by contrast, has all the products traditionally associated with major banks, such as savings and money market accounts. Based in Charlotte, N.C., Bank of America has the most branches of any U.S. bank: 5,800 in 29 states.