It wasn't just one person. There is a list of some of them at https://en.wikipedia.org/wiki/Atomic_spies https://en.wikipedia.org/wiki/Atomic_spies. Those are

It wasn't just one person. There is a list of some of them at https://en.wikipedia.org/wiki/Atomic_spies https://en.wikipedia.org/wiki/Atomic_spies. Those are the known knowns. There are also the known unknowns, the aliases from intercepted messages who were never matched to identities, and the unknown unknowns, the ones we haven't found out about. One of the spies on the Wikipedia list only had their identity revealed last year.