A federal judge in San Francisco said yesterday he will permit prosecutors in the espionage case against retired Navy communications expert Jerry Alfred Whitworth to use a major piece of evidence: a series of letters that the government contends Whitworth wrote offering to expose a "significant" spy network.

U.S. District Judge John P. Vukasin Jr. cited the "numerous common points between the espionage conspiracy" outlined in the letters, sent to the FBI's San Francisco office by a person identified only as "RUS", and "the known facts of the spy ring" masterminded by admitted spy John Anthony Walker Jr.

The ruling by Vukasin -- who earlier described the letters as "in essence a confession" and had refused to let prosecutors introduce them -- constitutes a major victory for the government. Opening arguments in the trial are set for Monday.

"Introduction of the RUS letters goes directly to what is the central issue, the defendant's alleged participation in the Walker espionage conspiracy," Vukasin said in a seven-page opinion.

He said that while permitting the jury to see the so-called "RUS" letters might be prejudicial to Whitworth, the last of four Navy men charged in the Walker spy case to face trial, "that prejudice cannot be deemed 'unfair.' "

In four letters sent to the FBI's San Francisco office between May and August 1984, "RUS" initially offered to expose "a significant espionage system" involving at least three other members in exchange for complete immunity, but the writer later withdrew the offer.

In the first letter, dated May 7, 1984, with a return address of "Somewhere USA," "RUS" claimed involvement "in espionage for several years" and "passed along Top Secret Cryptographic Keylists for military communications, Tech Manuals for same, Intelligence Messages, and etc." The writer identified a "contact" as having "been in 'the business' for more than 20 years and [planning] to continue indefinitely."

Whitworth, 46, a retired senior chief radioman, is charged with funneling to the Soviet Union highly sensitive Navy codes -- called cryptographic key cards and key lists -- and manuals for the design of the coding machines, through John Walker. Walker pleaded guilty Oct. 28 to spying for the Soviets since 1968 in a conspiracy that he said included his son, Navy Seaman Michael Lance Walker, who also pleaded guilty; John Walker's brother, retired Navy Lt. Cmdr. Arthur James Walker, who was convicted of espionage Aug. 9, and Whitworth.

In a hearing earlier this month, Assistant U.S. Attorney William S. Farmer Jr. argued that the facts detailed in the "RUS" letters "point to one person uniquely -- Jerry Whitworth" as the author.

Defense lawyers contended that prosecutors had failed to show conclusively that Whitworth wrote the letters, and that, in any case, the letters were a "virtual confession" that would interfere with Whitworth's ability to receive a fair trial.

"It's just too prejudicial," defense lawyer James Larson told Vukasin.

In the first letter, "RUS" claimed no knowledge that the material was being passed to the Soviet Union until a few years after the transfer began and that "since then I've been remorseful and wished to be free."

In the second letter, dated May 21, 1984, "RUS" told the FBI that "I feel that to come forward and help break the espionage ring would compensate for my wrongdoing, consequently clearing my conscience."

However, "RUS" said, "There are other emotions: the difficulty of ratting on a 'friend' and the potential of getting caught up in a legal mess."

"RUS" said the contact "thinks he has a good organization and has no real fear of being caught, less some coincidental misfortune."

The third letter declined an apparent invitation from FBI officials to meet with them. In the final letter, dated Aug. 13, 1984, "RUS" reported a change of mind about cooperating.

"To think I could help you and not make my own involvement known to the public, I believe is naive," "RUS" wrote. "I have great difficulty in coming forth, particularly, since the chances of my past involvement ever being known is . . . remote, as long as I remain silent."

RUS said the contact was "pressing for more material . . . . I haven't explicitly told him, I'm no longer in the business."