British codebreakers at Bletchley Park were not permitted to monitor, let alone decrypt, Soviet communications.

In June 1941 Josef Stalin received a warning from the nkvd, the forerunner of the KGB that a Nazi attack on the Soviet Union was imminent.

 “You can tell your ‘source’ in German air force headquarters to go fuck himself,” was the Soviet leader’s response. “He’s not a ‘source’, he’s a disinformer.” 

The invasion came a week later. 

The Economist 20 July 2023

Calder Walton’s “Spies” is a riveting history of espionage (economist.com)

Spies: The Epic Intelligence War Between East and West. By Calder Walton. Simon & Jchuster; 688 pages