MI5 then vetted Fuchs again when he returned to Britain after WW2 to work on British atomic research.
Vetting him caused disagreement within MI5— those worried about him being a Soviet agent were later proved correct
Here is Fuchs’ eventual espionage confession to MI5.
I’ve never read anything like it— check it out.
He describes how he used “controlled schizophrenia” to separate his mind into two compartments, one for his espionage for the Soviets, the other for all else.