Secretary of Defense Caspar Weinberger, citing the importance of the telephone system to defense posture, has asked the Justice Department to drop its six-year-old antitrust case against American Telephone & Telegraph Co.

In testimony before a Senate committee and in a letter to Attorney General William French Smith, Weinberger reiterated a long-standing Defense Department position, opposing a Bell System break up. But previous Defense Department officials have never publicly urged dismissal of the case.

Smith and his top deputy, Edward Schmults, have removed themselves from the case because of ties to AT&T.

A spokesman for William Baxter, head of the Justice Department's antitrust division, said that no response to the letter had been sent. But the Weinberger letter, which Justice and Defense department spokesman said is "classified," and testimony, which was presented to the Senate Armed Services Committee on March 23, raise the visibility of the Defense Department's posture on the issue.

"Because of the discussions I have had concerning the effect" of the suit which would break up the Bell System, "I have written to the Attorney General and urged very strongly that the suit be dismissed, recognizing all of the problems that might cause and because of the fact it seems to me essential that we keep together the one communications network we now have, and have to rely on," Weinberger told the Senate panel.

A Defense Department spokesman said that AT&T holds contracts totaling $597.77 million in the current cefense budget. "They provide a vital link as far as command and control systems," the spokesman said, referring to the defense communications network.

Herbert Jasper, spokesman for the Ad Hoc Committee on Competitive Telecommunications, a lobbying group representing AT&T competitors, took a different view of the Defense Department view. "The truth of the matter is that Defense gets hundreds of millions in free services from AT&T and they like it," Jasper said. "It's that simple."