Allan Nunn May, 91, a British physicist who was among the first of the World War II "atomic scientists" to be exposed as Soviet spies, died Jan. 12 in Cambridge, England. The cause of death was not reported.

By the time Britain entered World War II, Dr. Nunn May was working at a secret site in Suffolk, England, on the British radar development team. In 1942, he was recruited by the legendary British physicist James Chadwick to work on the "Tube Alloys" project, Britain's secret atomic bomb team.

In 1943, Dr. Nunn May was transferred to Montreal and became part of the Manhattan Project to build an atomic bomb. That same year, he made contact with an agent of the GRU, Soviet military intelligence.

At his trial, he said he had wanted to warn the Soviets about the possibility that the German atomic program was producing fissionable material that could be used to construct a "dirty bomb." Such a bomb, even without producing a nuclear explosion, could flood an area with deadly material.

Under the code name "Alek," he passed atomic secrets and materials, including technical information on the Trinity (the first atomic explosion in New Mexico) and Hiroshima atomic bombs. He also reported to the Soviets the amount of nuclear material produced in Oak Ridge, Tenn., and Hanford, Wash.

He was finally unmasked by the defection of a cipher clerk at the Soviet Embassy in Ottawa. Igor Gouzenko went to the Canadians in 1946 with dispatches and ciphers that were a treasure trove of information concerning Soviet espionage in the Western Hemisphere and Soviet penetration of the atomic bomb project. This information led to the unmasking of Soviet spies and their accomplices.

Dr. Nunn May always maintained that he had given information to an ally, not an enemy. The only pay he received from his GRU handler was a bottle of whiskey and $200 in cash. Insulted, the physicist burned the money. He said his motives were preventing the United States from maintaining a nuclear monopoly and preserving "the safety of mankind."

He was brought to trial in Britain, pleaded guilty to violation of the Official Secrets Act and was sentenced in 1946 to 10 years of hard labor. He sewed mailbags, tutored fellow prisoners and worked as a clerk before his 1952 release from prison.

During his years in prison, some fellow scientists remained on friendly terms with him, even seeing that his research was published. Upon his release, Dr. Nunn May caught up with advances in theoretical physics and worked for a time in a Cambridge scientific laboratory. From 1961 to 1978, he was a research professor in Ghana, working on solid-state physics. In 1978, he returned to Cambridge.

Dr. Nunn May, who was born in Birmingham, England, received a first in physics from Cambridge University's Trinity Hall. He also received his physics doctorate from Cambridge.

In the 1930s, he was a researcher and professor at the University of London's Kings College. He also joined a communist organization and the Association of Scientific Workers.

During his early war work in Suffolk, he began concentrating on the use of photographic methods to find and follow fast particles from radioactive material. From that, he joined the Chadwick team to research the use of heavy water in the construction and operation of a nuclear reactor.

It was at this time that he read a secret U.S. report predicting that the Germans could use fissionable materials as poison against the enemy. The report assumed that the Germans were much further along in their atomic program than they ever were.

Survivors include his wife, the former Hilde Broda, of Cambridge; a son; a stepson; and five grandchildren.