In a simple, short statement yesterday, U.S. District Judge Harold Greene ordered the resumption tomorrow of the government's landmark antitrust suit designed to break up American Telephone & Telegraph Co.

In a Jan. 30 order, Greene responding to a request from the two sides, had ordered the Justice Department and AT&T to come up with a consent decree by 4 p.m. yesterday or continue the trial.

The decision yesterday follows by a week a letter to Greene from former Assistant Attorney General Sanford Litvack, who said the gvoernment and AT&T could not reach a settlement of the case by Greene's deadline.

The negotiations were complicated by the Reagan administration's failure until several days ago to nominate a successor to Litvack -- a Carter administration holdover until his departure last week. Stanford professor William Baxter has been named to succeed Litvack and presumably will supervise any further negotiations if they are reopened.

Whether settlement in the case can be reached is another question, as the two sides appear to have different views about whose move is next.

"The government is prepared to try the case and present its evidence," Gerald Connell, the Justice Department's lead attorney in the case, said yesterday.

Connell said no further settlement negotiations are scheduled. "If at some time they [AT&T] wish to continue settlement negotiations, we are prepared, but it's up to them to make the call," he said.

AT&T spokesman Pickard Wagner said Connell's statement is "news to me. It takes two people to resume a discussion.

"We believe that a final settlement is reachable in a few days, but without anyone in place, according to the Litvack statement, it doesn't make any sense to continue to talk without an agreement in view, particularly because of the cloud of uncertainty the negotiations had placed over our financinal plans," Wagner said yesterday.

The six-year-old case, which may last two more years, revolves around government charges that the Bell System violated antitrust laws during the late 1960s and early 1970s, particularly in its relationships with firms newly authorized by the federal government to compete in telecommunications fields.

The government has said it will ask Greene to order AT&T to divest itself of some or all of its 23 operating companies and its long-distance and equipment manufacturing divisions.

When the trial resumes tomorrow, the government intends to focus on evidence dealing with telephone terminal equipment business. According to a Jan. 26, 1981, filing with Greene, the Justice Department will try to "establish the general history of the defendants' foreign attachment tariffs."

The issue revolves around the AT&T rate schedules filed with the Federal Communications Commissions. The government's first witness will be Robert Feiner, who was involved with "Phonemaster" telephone equipment.