A federal court judge yesterday extended an antitrust decision that cost American Telephone and Telegraph Co. $276 million, ruling that many other parties injured by the illegal practices can seek damages.

According to the decision here by U.S. District Judge Thomas P. Jackson, the ruling of the federal court in New York can be applied by all affected telephone users between 1970 and 1978, who now only have to prove to the court how much they were damaged.

The extent of the potential liability facing AT&T was unclear, though some experts estimated it could run into the billions of dollars.

Jackson stayed the ruling for 10 days to give the Court of Appeals time to act. AT&T spokesman Pic Wagner said the company would appeal the ruling as well as continuing its appeal of the New York decision.

The case stems from an antitrust suit by Litton Industries charging AT&T forced it out of the telephone equipment business by requiring the use of special equipment to connect non-Bell switchboards to its system. Litton's 1981 victory was upheld on appeal in February.

The judge in that case ruled that the special equipment was not needed to protect AT&T's telephone network, as was claimed, but instead was used to maintain AT&T's monopoly power.

Under Jackson's ruling, users who paid the Bell System for that connecting equipment can now obtain damages on the basis of the Litton decision without going to trial on any issue but the amount of damages. Jackson indicated many of those affected have not yet been told they have a case against AT&T.

The judge specifically rejected AT&T's contention that it had uncovered new information that could have changed the verdict in the Litton case--the suggestion that Federal Communications Commission officials showed an "anti-AT&T bias" designed to help competitors and the Justice Department pursue antitrust cases against the Bell System.