Wilfrid Basil Mann, a little-known 71-year-old scientist-bureaucrat at the National Bureau of Standards, became -- much to his displeasure -- something of an international celebrity last week as a purported spy.

Mann is by all accounts a quiet, modest, unassuming man, tall and a little stoop-shouldered. He lives on a placid street in Chevy Chase. Sometime he walks his dog and exchanges minor pleasantries with his neighbors. sHe has served on his local PTA. He has a wife and three grown children. He has held the same job for 28 years, as chief of the Bureau of Standards' radioactivity section.

In the world of nuclear physics, which is so small that everyone in it knows everyone else, Mann is regarded as a highly competent if unspectacular experimenter who has made some significant contributions to the precise measurement of radiation levels. He is now president of the International Committee for Radionuclide Metrology.

There is only one thing remarkable about him: that, according to one British journalist and one member of Parliament, he is a former espionage agent for the Soviet Union, once a member of the spy ring that included Anthony Blunt, who yesterday admitted to years of spying for the Soviet Union.

Mann himself is not talking to the press. The Department of Commerce, which runs the Bureau of Standards, issued a statement last week saying that it has "no question concerning his loyalty to the United States." Before deciding not to talk to reporters, Mann gave an interview to a London Sunday Times reporter, who says Mann stoutly maintained his innocence. There is no evidence against him, but he has suffered embarassing damage to his reputation.

His troubles began with the publication in Britain of a book called "The Climate of Treason," by Andrew Boyle, which detailed the activities of two Englishmen on behalf of the Soviet Union.

Boyle referred to the two only as "Maurice" and "Basil," but last week Prime Minister Margaret Thatcher told Parliament that Maurice was Blunt. pThe announcement caused a tremendous uproar in Britain.

Then The New Statesman, a British weekly political magazine, printed a short item saying, among other things, that Basil was Dr. Wilfrid Mann, "a modestly obscure physicist." A few days later a member of Parliament demanded an investigation, reporters started calling, and Mann's orderlyy life temporarily went haywire.

In Boyle's book, Basil is described as "a pleasant Englishman of homosexual bent, a gifted physicist." Boyle says Basil, while working in Washington in the late 1940s, helped a Soviet spy name Donald Maclean, who was planted in the British Embassy, interpret for the Soviets information Mann had on the latest developments in nuclear physics, which without Basil's help would have been Greek to Maclean.

But before long, says Boyle, two Israeli intelligence agents passed on to James Angleton, then an agent of the Office of Strategic Services, later head of the covert side of the CIA, the information that Basil was a spy. Confronted with this, Boyle says, Basil "broke down quickly, confessing that he had become a covert communist in his student days and a secret agent for the Soviets not long afterwards."

Angleton, read this passage from the book this week and asked whether it is correct, said, "The answer's no. I stand by the statement issued by the United States Department of Commerce."

Boyle says that after Basil confessed, he agreed to change sides and become a double agent in return for "guarantees of protection and the promise of American citizenship," after which he both spilled the beans on Maclean and cheerfully provided Maclean only with useless or wrong information for passage to the Soviets.

Mann was attached to the British Embassy in Washington from either 1948 or 1949 to 1951 as a nuclear physicist, after which he went to the Bureau of Standards to take the job he holds today, measuring the radioactive output of various products, particularly for medical uses. He applied for and got U.S. citizenship in 1959. Other than that, the only evidence against him is that his middle name is Basil.

"I can't believe there's any foundation to the whole blooming ruckus," says Lauriston Taylor, who hired Mann at the Bureau of Standards back in 1951. "He did a magnificent job. I know his family well. You'd have to know him to know how absurd the idea is that he'd do something like that."

Taylor says that Mann underwent a stringent security check when he joined the Bureau of Standards and received a top-level "Q" clearance from the Atomic Energy Commission.

Several American scientists who worked with Mann on the development of the cyclotron at Berkeley in the 1930s remember him as a shy, pleasant, cooperative man, and as totally apolitical.

"There were lots of left-wing groups around Berkeley at that time," says Luis Alvarez, then a young physicist and now a Nobel laureate and professor emeritus at Berkeley, "and I never heard Wilfrid express any interest in them. He was just doing his job."

Mann's lawyer, Marsha Swiss, says Mann will tell his side of the story when the British government issues a statement clearing him, as the American government has done. A British Embassy spokesman said yesterday no such statement is in the works, and on Monday Britain's attorney general turned down a member of Parliament's request that he investigate Mann, saying there was no evidence to justify looking into Mann's history.

There is no firm evidence that there ever was a Basil. But in Boyle's account, Basil was the "fifth man" in a spy ring that included Blunt, Maclean, a flamboyantly decadent friend of MacLean named Guy Burgess, and the former head of the Soviet section of British Intelligence, H.A.R. Philby, known to his many friends as Kim.

The London Sunday Times reporter who interviewed Mann made available to The Post yesterday his notes from the interview, which he has not yet published. Mann told the reporter, David Leitch, that the charges that he was a spy are "completely unwarranted by the facts." Mann said he was not given U.S. citizenship in exchange for acting as a double agent, that he had never met Maclean, and that he "certainly was not a homosexual."

Mann told Leitch he had known Burgess and Philby, and he had become good friends with Angleton.

"I often enjoyed his magnificent orchids," Mann said. "I knew roughly what he did, and respected his ability and his dedication, but I never worked directly with him."

There are, however, a couple of inconsistencies in the story Mann told Leitch. In "American Men and Women of Science," Mann is listed as having started to work for the embassy in 1948. He told Leitch that he started in 1949.The year is important, for by 1949 Mclean had left Washington for Cairo.

And Taylor, who hired Mann at the Bureau of Standards, told the Post that Mann's security check had shown that he knew Maclean. Informed last night that Mann denied ever knowing Maclean Taylor acknowledged that his memory might have been faulty on that score.

Blunt, burgess, Maclean and Philby all attended Cambridge University -- which Mann did not -- and all became communists and spies in the 1930s. All held fairly important jobs in the British government. Philby, as Britain's chief liaison with the CIA and FBI, was a prominent figure around Washington in the late 1940s; he and Angleton used to lunch regularly at Harvey's restaurant on Connecticut Avenue.

In May 1951, MacLean and Burgess defected to the Soviet Union, just as MacLean was about to be apprehended for spying. Philby fell under suspicion, but officially was cleared in 1955. Then he, too, defected to the Soviet Union in 1963.

For years thereafter there had been speculation that a "fourth man" tipped off Burgess and MacLean, and Boyle says that man was Blunt. Blunt denies this, while admitting that he was a Soviet spy. Even connoisseurs of the conspiracy had not suggested the existence of a "fifth man" until Boyle brought forth the idea.

Boyle says there had to be a Basil to alert the CIA to MacLean. However, Ray Cline, former deputy director for intelligence at the CIA, says: "I think MacLean was fingered by another CIA source."

Cline says the only purpose a fifth man could have served was a "cut-out" -- conduit of information from Philby (the third man) to Blunt (the fourth man) whose function would have been to make the trail from Philby to Maclean fainter.

All of this speculation is great sport for spy buffs, but not for Wilfrid Mann. When a reporter knocked on his door Monday night, his wife answered and said her husband didn't wish to speak to the press. She explained that "a good friend from Paris" was visiting and the Manns wished to have a quiet social evening.

Then Mann got up from his chair in the living room and walked a circuitous path toward the door -- so as to stay mostly out of the reporter's line of eight -- until he stood next to his wife, still out of view. Through the door a tweed-encased arm could be seen handing a piece of paper to Mrs. Mann, and she handed it to the reporter.

"The U.S. Department of Commerce has declared its full confidence in my loyalty to the United States," the message read, "and I am awaiting an equally forthright statement from the British Government acknowledging my loyalty to that Government during the period of my service to them."