The U.S. Court of Appeals for the District of Columbia Circuit yesterday ruled that American Express was wrong to cancel a University of Maryland professor's credit card in l982 for a restaurant meal that had already been consumed. The decision could provide consumers with new protection against credit card cancellations,

A credit card "can be revoked without cause and without any waiting period, but it cannot be revoked for transactions that have occurred," the three-judge panel said. Their unanimous l9-page decision overturned a ruling by U. S. District Judge Harold H. Greene who had rejected a complaint by Oscar S. Gray, 57, a U-Md. law professor who lives in Washington.

The case could have far-reaching implications for American Express and its 14.2 million U.S. cardholders. Other credit card companies also could be affected, if the ruling is broadly applied.

Gray filed the case against American Express after taking his wife out to dinner on April 8, l982, for a l5th anniversary celebration at 209 1/2, a Capitol Hill restaurant at 209 1/2 Pennsylvania Ave SE. After "a lovely evening", Gray offered his American Express card to pay for the meal. But the restaurant, after checking with American Express, told Gray that the credit card company had refused to accept the charges for the meal and had instructed the restaurant to confiscate and destroy his card.

When Gray got on the telephone to talk to the American Express representative, he was informed, "Your account is canceled as of now."

That cancellation occurred at a time when Gray and American Express disagreed about a billing for some airline tickets. Under the Fair Credit Billing Act, the cardholder is protected from cancellation when there is a billing dispute.

But Gray and his attorney, Hubert H. Margolies, said yesterday that the court ruling does more than reaffirm consumer rights against cancellation during a billing dispute.

"The court is saying that if there has been a billing dispute that you are protected against cancellation, in keeping with the law," Gray said. But, in addition, he said, the court is saying that even when there is no billing dispute American Express cannot cancel transactions which have occurred prior to notification of cancellation.

American Express officials yesterday said it would be premature to comment on the case, since their attorneys still are studying the ruling. But a company representative, Walter Montgomery, expressed skepticism that the court decision prevents the company from canceling cards without prior notice.

The case has been remanded to the District Court for further proceedings consistent with the ruling.

In the opinion written by Judge Abner J. Mikva, with Judges Malcolm Richard Wilkey and Robert H. Bork concurring, the panel took note of American Express' concern that the ruling could result in "a great risk thrown on the credit card business."

But, the opinion said, "We think they protest too much. Within the limits of state and federal statutes, credit cards can still be canceled without cause and without notice. But the cancellation can affect only transactions which have not occurred before the cancellation is communicated to the cardholder. In practical terms American Express will have to make an effort to communicate its cancellation decision to the cardholder. The effort may be as informal as a phone call or a telegram."

The court also reprimanded American Express for the "extraordinary use of interrogotaries" and said that "only a lawyer such as Gray , tenacious even beyond the professional custom, would have been able to withstand the expense and excesses of this litigation."

Hundreds of pages of questions and answers were involved, the court said, and "The length, scope and detail of the interrogatories propounded by American Express suggest a strategy of attrition rather than a legitimate discovery of the facts needed to resolve a dispute over the account."