NEW YORK – American Express announced its first deal on Thursday to allow a major American financial institution – MBNA Corp. – to issue its credit cards in the United States.

American Express said MBNA will issue American Express-branded credit cards starting later this year, allowing the company to reach a much larger potential customer base. New York-based American Express will also continue to issue its own cards.

No financial terms were disclosed.

The alliance was made possible by a 2001 ruling in an antitrust case brought by the Justice Department. A federal court in Manhattan ordered Visa and MasterCard to drop rules that prohibited their member banks from also issuing American Express and Discover cards. Legal appeals by Visa and MasterCard have all been rejected.

MBNA already issues Visa and MasterCard credit cards. As a result of the alliance with American Express, it will become the nation’s first financial institution to participate in all three major card networks.

MBNA, which is headquartered in Wilmington, Del., bills itself as the nation’s largest independent issuer of credit cards, many of them distributed through professional and university affinity programs. It has several Maine locations and employs about 4,500 people in the state.

Shares in both companies finished up Thursday on the New York Stock Exchange. American Express rose $1.20, or 2.4 percent, to close at $50.95, while MBNA advanced 40 cents, or 1.5 percent, to $26.90.

According to data from CardWeb.com, American Express is the top card issuer by charge volume in the United States, with a 12.4 percent market share at year’s end. Citibank is second at 11.7 percent, and MBNA is third at 8.7 percent. When ranked by outstanding balances, the Citibank portfolio is on top, followed by MBNA and Bank One.

American Express had made it clear since the 2001 court ruling that it was seeking U.S. distributors.

“This is a milestone agreement for us,” American Express chairman and chief executive Ken Chenault said Thursday in an interview with The Associated Press.

He compared the court ruling that forced Visa and MasterCard to allow competition to the ruling that led to the breakup of AT&T, creating the “Baby Bells.”

“Now there will be three networks actively competing,” Chenault said. This, he added, should spur innovation in the industry.

Bruce L. Hammonds, MBNA president and chief executive, said the deal will result in more choices for customers, including credit cards with rewards offerings from American Express.

David Balto, an antitrust lawyer who formerly worked at the Federal Trade Commission, said he would not be surprised if American Express seeks more relationships with U.S. banks. The reason, he said, is that while American Express has strength in credit and charge cards, it has no debit cards – a fast-growing segment of the market.

“I think they’ll look for relations with banks that have deposit accounts to give them a role in the debit market.”

Robert McKinley, chief executive of CardWeb.com, an online publisher of information about payment cards, said the alliance of American Express and MBNA would change the industry.

“The fact that American Express could land the No. 2 player in the country is stunning,” he said. “It means the landscape will change sooner rather than later.”

In a statement, MasterCard derided the American Express-MBNA alliance, saying it “indicates that they think an Amex card issued by another bank would be more attractive to consumers than one issued directly by American Express itself.”

MasterCard also said it plans to appeal the antitrust case to the Supreme Court and expects to win, leaving American Express and MBNA unable to complete their deal.