A Pentagon investigation to determine the source of the leak of Defense Secretary Caspar W. Weinberger's controversial, but unclassified, presummit letter and Soviet arms violation report to President Reagan will be limited to Defense Department personnel, a Pentagon spokesman said yesterday.

Assistant Secretary of Defense Robert B. Sims added that the inquiry will be based on a "breach of trust" regulation rather than a violation of the law.

Sims said Weinberger had told subordinates, including Assistant Secretary Richard N. Perle, who directed the study, that they were not to discuss or release the letter or the accompanying 11-page executive summary, which Weinberger delivered to Reagan on Wednesday.

The letter, which urged the president not to agree to continued adherence to the SALT II arms-control agreement and not to accept limitations on his Strategic Defense Initiative, became the center of a presummit uproar after it was leaked Friday to The Washington Post and The New York Times and was published Saturday.

The stories took on a more sensational coloration in Sunday's papers after White House spokesman Larry Speakes described the president as angry about the leak. In addition, a senior administration official traveling to Geneva Saturday morning on Air Force One was asked by a reporter if the leak "was intended to sabotage the summit," and responded: "Sure it was."

Yesterday an administration official here identified the senior official as national security affairs adviser Robert C. McFarlane.

After Sunday news accounts with the "sabotage" quote aggravated the incident, McFarlane was asked on NBC's "Meet the Press" if he had been the senior administration official quoted. He did not respond directly and downplayed the importance of the incident.

Later on Sunday, Reagan was asked about the senior official who had agreed that leaking the Weinberger letter was an attempt to "sabotage" the summit. Reagan replied, "I'm wondering if that individual is not a figment of someone in the press' imagination."

At the Pentagon yesterday, Sims said copies of the letter, the executive summary and the 70-page classified report were sent Wednesday to McFarlane, White House chief of staff Donald T. Regan, Secretary of State George P. Shultz, Central Intelligence Agency Director William J. Casey and Kenneth L. Adelman, director of the Arms Control and Disarmament Agency -- key members of the National Security Council, which ordered the study last June.

Sims added that Weinberger had told Perle and Undersecretary Fred C. Ikle that it "was for the president and the White House to decide" about releasing the material.

It is routine within the bureaucracy to make copies of such controversial documents in order to gather comments. That practice was facilitated in this case, according to one source with access to the material, by the fact that the letter and the executive summary were unclassified. One source said the Pentagon "was almost asking for a leak" by not marking it secret.

The investigation of the leak, ordered Sunday by Weinberger, "should not take long," Sims said. The results of the Pentagon inquiry will be forwarded to the Justice Department, he said, and officials there will decide the next step.

The individual who last Friday provided The Post with Weinberger's letter and the executive summary refused yesterday to discuss for publication the reasons for that action. The Post source continued to ask that neither his or her name nor place of work be mentioned.

Speculation on the source of the leak, which originally focused on the Pentagon and individuals who back publicizing a tough U.S. stance on Soviet arms violations, broadened yesterday to include the possibility that the goal may have been just the opposite -- to embarrass Weinberger or discredit his report.