A brother of accused spy John Anthony Walker Jr. admitted to FBI agents last week that he had given Walker sensitive defense information for delivery to Soviet agents beginning in September 1980, according to papers issued for the brother's arrest yesterday on espionage charges.

The brother, retired Navy Lt. Cmdr. Arthur James Walker, 50, was taken into custody by federal agents about 7 last night at his Virginia Beach home, FBI Director William Webster said.

According to an affidavit supporting the government's complaint, Walker, who works for a defense contractor, admitted to FBI agents last Friday that "on a number of occasions" he "turned over to his brother John Anthony Walker Jr. documents, files, photographs, booklets and defense plans relating to United States Naval forces, knowing that John Anthony Walker Jr. intended to deliver or transmit these items to 'the Russians.' "

The affidavit states that Arthur Walker, who holds a "secret clearance" at VSE Corp. in Chesapeake, Va., admitted receiving $12,000 for the material he furnished to his brother, described in the affidavit as an "agent of the Soviet Union."

Both John Walker, 47, and his son, Michael Lance Walker, 22, a seaman on the USS Nimitz, have been charged with espionage. A federal magistrate in Baltimore yesterday ordered Michael Walker held without bond.

While in the Navy, where he served from 1953 through 1973, Arthur Walker held a "top secret clearance," the affidavit states. Walker is not charged with espionage activity during his Navy service.

Arthur Walker is scheduled to appear before federal Magistrate Gilbert R. Swink in Norfolk this morning, according to the FBI. They would not say where he was being held.

Reached at the family's Virginia Beach home last night, Arthur Walker's son, Curt, 23, said, "We have nothing to say right now. Thank you for calling."

FBI officials had said earlier that they expected several additional arrests in the Walker case, but spokesman Ed Gooderham said last night that he could not say whether any more arrests would be made.

He said he could not explain why Arthur Walker was arrested last night when he allegedly had admitted his involvement to FBI agents last week.

John and Michael Walker were arrested last week. According to an affidavit filed in federal court in Baltimore, the arrests were made after FBI agents saw the elder Walker leave a bag filled with classified documents at a remote site in western Montgomery County. Both father and son were indicted Tuesday on espionage and conspiracy charges.

Soviet Vice Consul Aleksey Tkachenko, who has since left the country, was spotted in the area, according to the indictment.

The affidavit supporting Arthur Walker's arrest states that on or about April 28, 1982, Arthur Walker gave his brother a defense file from VSE labeled "CASREP Extract File Number 2" and permitted John Walker to photograph part of the file.

Arthur Walker, who has worked for VSE since 1980, admitted that later the same day he acquired "another chunk of these, keeping a chunk in my briefcase," and photographed that material himself in his brother's business office, according to the affidavit.

The affidavit states that Arthur Walker told the agents he left the camera and file on his brother's desk with a note.

According to the affidavit, "Arthur James Walker admitted that he knew the information in the CASREP file related to the national defense and that it involved five ships of the United States Navy. He also admitted that he received approximately $12,000 from his brother as payment for the material he furnished."

The affidavit does not specify what material was contained in the CASREP file, although it states the file was classified "confidential." A senior vice president of VSE said last night that CASREP refers to casualty reports on such things as malfunctioning machinery.

Among the information found in the bag of classified documents left in Montgomery County by John Walker, according to the affidavit, were "coded references to other espionage associates of John Walker, including one person designated as 'K' -- described as a male involved in carrier and amphibious ship maintenance planning."

A search of John Walker's home, business, boat, airplane and cars located additional documents also referring to "K," and "more fully identified that person as having the name of Art," the affidavit states.

"Further investigation by FBI agents and Navy officials confirmed that Arthur James Walker . . . was the person most probably identical with 'K' " based on his relationship to John Walker, his naval service and his employment at VSE, according to the affidavit.

His duties at VSE involve U.S. Navy carrier and amphibious ship maintenance planning, the affidavit states.

F.J. Schiavi, a senior vice president of VSE, described the Alexandria-based company as an engineering firm of 1,700 employes that does substantial work for the Navy and other military agencies.

Ron Howell, engineering site manager for VSE's branch in Chesapeake, said Walker had been employed there for more than five years. He said he was hired on the basis "of his military experience" and seemed like "a pleasant nice fellow, Mr. Average. Just like your next door neighbor."

He said Walker's job was to develop plans for the repair of Navy ships in the section devoted to "amphibious planning and engineering repair and analysis."

Howell said he couldn't conceive of any information that Walker would have had access to through his job that would be of interest to the Soviets.

"There's nothing he could get out of the firm that he couldn't get in the public library," Howell said.

Thomas J. Scarfato, who said he had lived two doors away from Walker for 15 years, described him as "a very nice fellow, outstanding, and I'm being sincere about it. I'm very, very surprised."

He said Walker was always offering gardening tips and frequently helped him with various mechanical problems with his car and the electrical pump on his swimming pool. "If I wasn't quite sure, he would drop everything and come right over," he said. "An ideal neighbor."

In arguing that Michael Walker should be held without bond, Assistant U.S. Attorney Michael Schatzow disclosed that Walker had allegedly taken "stacks of classified documents" to his father John on 10 occasions.

He said the documents weighed a total of 20 pounds, and that another 15 pounds of classified material was found near Walker's bunk on the Nimitz.

Schatzow said that while Michael Walker was working in a clerical position at a fighter squadron at the Oceana Naval Air Station in Virginia Beach, where he was assigned beginning in April 1983, he mentioned to his father that he had seen some classified documents.

John Walker told his son that "he could make some money" if he brought the document to him, Schatzow said.

"He thereafter took a number of documents to his father," and later, in about March 1984, John Walker paid his son $1,000 in return for the information, he said.

State Corporation Commission records list Arthur Walker as secretary and treasurer of two of his brother's private detective and "debugging" companies, Confidential Reports Inc. and Associated Agents Inc. Both operate from a suite in a Virginia Beach office building.

But Laurie Robinson, part owner of Confidential Reports, said the records are at least partly incorrect. She said she was vice president, secretary and treasurer of Confidential Reports, and Arthur Walker was only employed as an investigator. "He is not a corporate officer in any way, shape or form."

She said he had worked for the firm for almost four years.

Arthur and John Walker formerly were partners in a Virginia Beach electronics business called Walker Enterprises.

After receiving submarine training, Arthur Walker served aboard a number of submarines, rising to the rank of lieutenant commander.

From 1968 on, he was assigned to the Atlantic Fleet Tactical School as an antisubmarine warfare instructor, according to his Navy biographical sheet.

Married to the former Rita Clare Fritsch, Walker has three children, Andrea Jay, 28, Eric Paul, 26, and Curt Christopher, 23