Theodore Alvin Hall, 74, who as a 19-year-old prodigy helped develop the atomic bomb and was later said to have passed along the vital secrets of this work to the Soviet Union, died of cancer Nov. 1 in Cambridge, England.

Early in 1944, Mr. Hall was one of four Harvard physicists recruited to work on a top-secret atomic weapon for military use against Japan or Nazi Germany during World War II. Arriving at the Los Alamos, N.M., headquarters for the Manhattan Project -- the code name for the bomb building enterprise -- he was assigned to the division responsible for mastering the physics of the implosion techniques that would be used to detonate the bomb.

In a 1997 book, "Bombshell: The Secret Story of America's Unknown Atomic Spy Conspiracy," authors Joseph Albright and Marcia Kunstel said Mr. Hall gave Soviet agents a detailed description of the "Fat Man" plutonium bomb, which the United States had tested in the New Mexican desert and later dropped on the Japanese city of Nagasaki. From this information, the Soviets developed and tested their own bomb four years later. KGB archives opened after the 1989 collapse of the Soviet Union and declassified wartime cables between the United States and Moscow were among the sources for the material in the book.

According to Albright and Kunstel, the information given to the Soviets by Mr. Hall, with the assistance as a courier of his former Harvard University roommate, Saville Sax, preceded that supplied by Klaus Fuchs, the German-born Los Alamos scientist who later was convicted of spying for the Soviets. Evidence cited in "Bombshell" suggested that Fuchs only confirmed information the Soviets already had received from Mr. Hall. Damage to the Western Alliance resulting from Mr. Hall's information may have exceeded that resulting from the information supplied to the Soviets by the so-called Cambridge Comintern, whose members included the famed spies Kim Philby and Donald Maclean.

In 1947, according to Albright and Kunstel, Mr. Hall may have been the source of information that enabled the Soviets to build a hydrogen bomb.

In the early 1950s, Mr. Hall was interviewed by the FBI after his name was discovered in Soviet wartime codes that were deciphered by U.S. cryptanalysts. This code breakthrough eventually led to the espionage prosecution and execution in 1953 of Julius and Ethel Rosenberg for what then-FBI Director J. Edgar Hoover called "the crime of the century." But "Bombshell" suggests that the Rosenbergs were not major players in the theft of atomic secrets. "I was more responsible than they were," it quotes Mr. Hall as telling his Soviet control agent.

But no charges were ever brought against Mr. Hall. Robert McQueen, an FBI agent who interrogated him in 1951, told The Washington Post in 1996 that he "was convinced that Hall was guilty, but I could never develop enough evidence to prosecute him."

Last year, Mr. Hall told the British Broadcasting Corp. how his interrogation by the FBI ended: "I reached for my coat, I think, and I just got up and walked out. And step by step, waiting for the handcuffs to be put on before I walked into the elevator, expecting to be collared before I got on the elevator. But I called for the lift, and it came, and I went in, and I got in and went downstairs, and walked out onto the street. And they didn't come."

In 1962, he moved with his family to England, where he took a research position in biophysics at Cambridge University's Cavendish laboratory, where he was a pioneer in developing biological X-ray microanalysis.

A Soviet cable declassified by the U.S. National Security Agency in 1995 identified Mr. Hall and Sax as Soviet informants. In 1996 in two conversations with Michael Dobbs of The Washington Post at Mr. Hall's modest, two-story house in England and in later telephone conversations, Mr. Hall declined to answer questions about intelligence activity on behalf of the Soviet Union. He said he feared that a newspaper account of such a complicated episode would inevitably be "sensational" and that such material would best be handled "by an historian."

"These events or supposed events happened 50 years ago," he said. "If they are made public, there will be a certain amount of interest, but it will die down."

In a statement to Albright and Kunstel in 1997, Mr. Hall said he had been "immature, inexperienced and far too sure of myself" when he was working at Los Alamos.

"During 1944, I was worried about the dangers of an American monopoly of atomic weapons if there should be a postwar depression. . . . To help prevent that monopoly, I contemplated a brief encounter with a Soviet agent, just to inform them of the existence of the A-bomb project. I anticipated a very limited contact. With any luck, it might easily have turned out that way, but it was not to be."

Mr. Hall publicly confirmed his activities last year, his daughter Ruth Hall said, according to the Associated Press.

"I don't call it an admission. I call it an achievement," she said. "He wasn't a spy in the sense of someone who goes into it for glory or money."

Mr. Hall was born in New York and grew up there. He entered Harvard at age 16. Both he and his roommate Sax were active in communist youth organizations. As related in "Bombshell," Mr. Hall's Soviet handler was Sergei Kurnakov, a former czarist cavalry officer, who reported to his superiors that Mr. Hall "had an exceptionally keen mind and a broad outlook and is politically developed."

At one point the book said that Mr. Hall and Sax, fearing that the FBI might be hot on their trail, considered dressing up as women and walking into New York's Soviet consulate to seek asylum. They dropped the idea after Mr. Hall's wife, Joan, asked them whether they had ever tried walking in high heels.

Sax died in 1980. He had held various jobs during his working career, including a position as a teacher of "values clarification" in a federally funded education program. "Get in touch with your love feelings," he had written in a manual on meditation.

Mr. Hall's survivors include his wife, two daughters and three grandchildren.