Embattled veterans administrator Robert P. Nimmo resigned yesterday just ahead of the scheduled publication this month of a General Accounting Office report that is expected to criticize his use of a chartered military aircraft, first-class air travel and a personal chauffeur in violation of the law.

A spokesman for Nimmo said the Veterans Administration chief had not seen the report. But a source close to the GAO probe said that Frank Sato, the inspector general for the VA, had been fully briefed on its contents last Thursday. Nimmo was unavailable for comment late yesterday.

White House counselor Edwin Meese III said Nimmo called him the same day to tell him of his plans for resignation. Meese said Nimmo mentioned the forthcoming GAO report, but contended that it was not "the controlling factor" in Nimmo's decision.

"This was totally self-initiated by Bob," Meese said of Nimmo's action. "He felt that he had done as much as he could for the president and that it was time to step down."

Nimmo, a 60-year-old World War II veteran who has been under constant fire from Vietnam veterans for purported insensitivity to their problems, told the Associated Press earlier yesterday that his finances were the chief reason for quitting and that "compelling personal reasons" dictated his return to California.

Nimmo's government financial disclosure statement shows that he and his wife, Pat, own property valued at more than $750,000.

The resignation of Nimmo closes what has been a painful chapter for the Reagan administration in its dealings with veterans.

Nimmo has been a constant target of Vietnam veterans on substantive issues, such as the cutback of funding for veterans counseling centers, and on symbolic ones, such as his refusal to allow VA employes to attend groundbreaking ceremonies for the Vietnam Veterans Memorial.

On one occasion he compared the effects of the defoliant Agent Orange to the effects of "teen-age acne."

Two Vietnam veterans resigned successively as deputy administrators because of their differences with Nimmo.

R. Jack Powell, executive director of the Paralyzed Veterans of America and a paralyzed Green Beret veteran of the Vietnam war, said that at a luncheon meeting last winter with seven leaders of veterans groups, including blind, disabled and paralyzed veterans, Nimmo referred to military service as "mere inconvenience."

Powell said this remark was typical of a series of "unfortunate remarks" made by Nimmo during his tenure.

The criticism of Nimmo was so intense that at one point White House chief of staff James A. Baker III urged that he be replaced. But President Reagan sided with his longtime counselor Meese, who defended Nimmo's performance.

Meese said yesterday that Nimmo was criticized chiefly because he was the author of "necessary economies" at a time there was an increasing demand for services and that the president would have kept the VA administrator if he had wanted to stay.

The statement echoed a defense Nimmo once made of his own record: "What has to be recognized, I think, by these people, if they want to be realistic, is there are more words than 'more, more, more.' "

But the harshest criticism of Nimmo was that he himself wanted "more" -- in one case admittedly more than the law allowed.

Last spring the Better Government Association, an independent government watchdog, accused Nimmo of making personal use of government money. Under pressure, Nimmo asked the VA inspector general to investigate the charges.

When inspector general Sato concluded that Nimmo had violated a 1981 law by using a chauffeur to drive him to and from work, Nimmo agreed to reimburse the government $6,441. He also agreed to terminate a $708-a-month lease for a 1982 Buick Electra, which is larger than the compact cars provided other agency heads.

Nimmo said he was making the refund on the advice of Sato, whose report also criticized Nimmo for spending $54,183 to redecorate his office and his decision to send his old office furniture to his daughter, Mary Nimmo, who serves as a Commerce Department spokesman.

The GAO report on which Sato was briefed last week went over some of the same ground and was described by one administration source as "an auditor's dream." But the GAO also broke new ground.

Joe Totten, group director of the GAO probe, said that it involved three areas that Sato's report did not cover. These include Nimmo's use of first-class air travel, annual leave and purportedly improper use of a military plane.

The report is scheduled to be formally delivered to Nimmo late this week. He will have 10 days in which to comment before the document is published within three or four weeks.

Nimmo's announcement was greeted happily by several congressional critics. Rep. Allen E. Ertel (D-Pa.), who had circulated a petition in Congress calling for Nimmo's ouster, said in a typical comment: "I believe the VA and veterans everywhere can only be better off with a new, understanding VA chief."

Meese said that Nimmo had agreed to serve until a new VA administrator can be found and said that he does not know what the timetable will be for a new appointment.

The Nimmo resignation caught high White House officials, except for Meese, by surprise.

Nimmo announced he was stepping down at a hastily called conference of VA executives where the only other speaker was the chaplain, who gave the benediction.