Early in the war, when he was back in England, he was offered an assistant's post at the atom-bomb development project at Birmingham University and signed a pledge of secrecy.

According to a statement that he later made at the War Office in London in 1950, as reported in the 1987 book ''Klaus Fuchs: the Man Who Stole the Atomb Bomb'' by Norman Moss, Dr. Fuchs said, ''When I learned the purpose of the work, I decided to inform Russia, and I established contact through another member of the Communist Party.''

In the statement, he added: ''Since that time I have had continuous contact with persons who were completely unknown to me, except that I knew that they would hand whatever information I gave them to the Russian authorities. At this time I had complete confidence in Russian policy and I believed that the Western Allies deliberately allowed Russia and Germany to fight each other to the death. I had, therefore, no hesitation in giving all the information I had, even though occasionally I tried to concentrate mainly on giving information about the results of my own work.'' To Los Alamos During War

Along with other British nuclear scientists, he proceeded to the United States during the war and took part, at Los Alamos, in the creation of the first atomic bomb.

He returned to Britain after the war and became an executive of the British nuclear energy research center at Harwell, outside Oxford.

At a preliminary hearing after he was arrested, a British prosecutor observed that Dr. Fuchs ''produced in himself a classic example of that immortal duality of English literature -a Jekyll and Hyde.''

''As Jekyll he was a normal citizen in the use of his magnificent brain in the cause of science,'' the prosecutor said. ''As Hyde, he was betraying his oath of allegiance, his vows of security and the friendship of his friends.''