American Telephone & Telegraph Co. is actively considering asking a federal court in New Jersey to modify a 1956 consent decree between the company and the government, which has barred the phone company from a variety of businesses it seeks to enter, The Washington Post has learned.

The controversial decree, which ended a major federal antitrust suit against AT&T and was signed over the protests of the government trial staff handling the case, has barred AT&T from competing in unregulated businesses, such as the data-processing field.

Word of the possible AT&T action comes one day after the Federal Communications Commission, in its so-called Computer II decision, voted in effect to modify the agreement on its own and allow the Bell System to compete in the computer business and in some auxiliary telephone services. Already, the FCC's interpretation of its authority to do so is the target of an appeals court proceeding by a major trade association.

"In light of the FCC's decision in the second computer inquiry, our lawyers are considering asking the U.S. District Court for the District of New Jersey to clarify the 1956 consent decree," a spokesman for AT&T said yesterday. "But we won't be making a decision on this step until we see the final text of the commission's order."

The decree prohibits AT&T and its operating companies from going into businesses other than "communications services and facilities . . . the charges for which are subject to public regulation."

The decree's significance lies in the fact that AT&T, with hookups to more than 130 million telephones and an overwhelming market share of most significant telecommunications markets, is viewed by some segments of the computer and data-processing industries as an overpowering threat to companies in those fields.

Further, industry observers say the fate of the consent decree is directly linked to the government's current massive antitrust case against AT&T, which is scheduled to go to trial early in 1981. Court action on the decree could be the first step in AT&T's seeking settlement negotiations with the government on that case, some experts said yesterday.

The future of the decree also has been the subject of a deep split within the Carter administration. In testimony before Congress, which has been considering legislation revising the terms of the consent decree, the Justice Department's antitrust division said it opposed modification of the decree and clashed with the National Telecommunications and Information Administration, which endorsed the revision of the decree.

Current and future AT&T competitors -- such as long-distance telephone firms and equipment manufacturers -- also have fought congressional action on the decree, and from those quarters reaction to the AT&T initiative was swift.

"We share AT&T's apparent perception that the FCC does not have the power to modify or interpret the 1956 consent decree," said Jack Biddle, president of the Computer and Communications Industry Association, the trade group that has filed the appeals court challenge the FCC move. "The courts have established strict requirements as a rpedicate for modification."

James Casserly, an attorney with Wilkinson, Cragun and Barker, a law firm representing the Independent Data Communications Manufacturers Associations, said he doubted that AT&T could get what it is seeking from the court.