The concentration of the nation's communications facilities in the hands of American Telephone & Telegraph Co. leaves the U.S. long-distance system highly vulnerable to nuclear and terrorist attack, a new study discloses.

In fact, a leading research firm, SRI International reports in the study that the loss of just 20 long-distance transfer facilities "destroys the entire long-distance telephone system in the United States."

The revelation of the findings in the February 1981 report comes as Defense Secretary Caspar Weinberger is actively urging the Department of Justice to drop its antitrust suit designed to split up AT&T.

In Senate testimony by Weinberger March 23 and in talks with Assistant Attorney General William Baxter, the Defense Department has urged that the case be dropped. Baxter has said he will not drop the landmark suit, but will consider Defense Department concerns if the government wins the liability side of the suit and moves into the relief phase.

Although Weinberger's active role in the matter has raised the visibility and potential political significance of the issue, Defense Department contentions about the importance of maintaining AT&T's system long has been a thorn in the side of some antitrust activists and competing communications industry representatives who for decades have sought to split up the Bell System.

The arguments were given so much weight in 1956 that the Defense Department, based almost exclusively on AT&T research, effectively forced the Justice Department to settle another antitrust suit against the giant phone company. That suit also was designed to force AT&T to divest itself of its long-distance and equipment arms.

A 1959 investigation of the 1956 consent decree by the House antitrust subcommittee revealed that the Defense Department had sent Justice lawyers a letter "ghost written" by AT&T and concluded that Defense officials "abdicated" to AT&T "their official responsibility."

Historically, AT&T, the world's largest company, and the Defense Department routinely have swapped personnel, and officials of the two institutions have developed long-standing and close relationships. But there has been little public airing of the sensative relationships and the multi-billion-dollar scope of AT&T Defense contracts.

Industry experts believe the new 234-page report is one of the most comprehensive studies of national defense communications issues ever assembled. It was prepared by SRI, which used to be known as Stanford Research Institute, a nonprofit company that employs 3,000 persons and is based in Menlo Park, Calif. A Washington spokesman for the company said officials would not comment on the study and would not discuss the backgrounds of the authors.

The Defense Department refused to permit interviews with communications-policy experts on the AT&T subject.

Ironically, AT&T representatives have circulated to key policymakers a study prepared for AT&T by another team of SRI consultants which warns of the dangers of splitting up AT&T.

This paper, 17 pages without footnotes or citations and published in January, says AT&T divestiture "will make it impossible to provide the essential system planning and engineering to adapt, enhance and modify the core network system for national security."

In an appearance before the Senate Armed Services Committee in March, Weinberger called the Bell System "the most important communication net we have to service our strategic systems," and said he feels it essential that "we keep together this one communications network we have."

But the new SRI report generally refutes the theory that the existing AT&T system effectively protects national communications needs. Instead, the study focuses on the patchwork governmental handling of communication policy and says that in terms of federal policy, responsibility for vital telephone links connecting the country is haphazard and unfocused.

In essence, the report is sharply critical of the government's virtual total dependence on private telephone concerns and is equally critical of the federal failure to develop a coordinated national communications policy.