British intelligence demonstrably contributed to Soviet victory on the Eastern Front, although not entirely as they would have liked. SIS (MI6) had traditionally fixated on the Soviet Union in the run-up to the Second World War, and even with the USSR coming in as an ally against Nazi Germany, the instincts in the British intelligence community were to use this new friendship as an opportunity to gather intelligence on the Soviets rather than for them.

As the United States was contributing in material terms to the Soviet war effort after 1941, there was a suggestion by the British Military Mission in Moscow that Britain contribute intelligence "as the best available method of aiding the Russians and demonstrating Britain's military prowess and important." (Sharing Secrets with Stalin: How the Allies Traded Intelligence, 1941-45, Bradley F Smith).

Enigma decrypts (Ultra) were the x-factor and there was a reluctance to let the Soviets in on the secret that German encryption had been broken, in part because Soviet signals security was so poor that the SD (the intelligence arm of the SS) were likely to discover the British were receiving encrypted German messages word for word:

"Various areas of the Soviet radio system were characterized by the above-mentioned rigid adherence to established procedure which facilitated the German radio intelligence effort. For instance, several code designations used in both CW and phone communication remained unchanged for years along the entire Russian front." (German Radio Intelligence, Albert Praun)

Enigma decrypts (Ultra) were the x-factor and there was a reluctance to let the Soviets in on the secret that German encryption had been broken, in part because Soviet signals security was so poor that the SD (the intelligence arm of the SS) was likely to discover the British were receiving encrypted German messages word for word: and widespread the British codebreaking operation was... but it scarcely mattered as MI6 had been living rent-free in Stalin's head for decades.)

Ultimately, Ultra did play a key role in turning the tide on the Eastern Front: double agent John Cairncross (who was stationed at Bletchley Park) supplied his KGB handlers with raw decrypts of traffic between Berlin and Army Group South that revealed the entire German order of battle for what became Battle of Kursk. The last significant German offensive in the east was turned back by the Red Army, signalling the beginning of the end. (Colossus: The Secrets of Bletchley Park's Code-Breaking Computers, B. Jack Copeland)