There can be little doubt that the GRU had regarded Sonya's partnership with Melita Norwood as critical, enabling them to secure vital information on the metallurgy of uranium and nuclear reactor design. When Melita was first exposed by Vasili Mitrokhin and Christopher Andrew in 1999 some doubt was raised by commentators as to her actual significance as a spy. Some questioned whether the GRU would have put such an important spy as Sonya at risk by allowing her to work with an agent of such little significance as Melita Norwood. The notes taken from Melita Norwood's file by Mitrokhin are also cautious on this point. They ‘record that she was controlled from 1941 to 1944 by an unidentified “head agent” codenamed FIR. The fact that FIR was also “involved in the Klaus Fuchs case” and was questioned after the war by MI5 strongly suggest, but do not quite prove, that FIR was the Centre's codename for sonya of the GRU.’ Sonya's address, The Firs, Oxford, would suggest that FIR was indeed Sonya. This has been more or less verified by the Russians in an official history of the GRU published in 2004, which states that Letty ‘began passing information on the Tube Alloys project to her controller (“veroyatno, eto bila Ursula Kuchinski “ (probably, this was Ursula Kuzcinski)) from September 1941’. ( Veroyatno can be taken as GRU-speak for nesomnenno, which translates as ‘undoubtedly’.) That Melita was crucial to the development of the Soviet atomic project from 1941 onwards is also suggested by the steps taken to shelter her from detection.

Hitherto, it has been thought that when in 1944 Melita was moved from Sonya's control back to the Centre it was solely because of the fierce rivalry between the GRU and the NKVD. Beria, the head of the NKVD, had only recently been told by Stalin to gather all his atomic agents under one roof. However, the GRU's official history points to an agreement between Moscow Centre and the GRU regarding Sonya. Both were concerned at this time that she had been compromised following the arrest of her first husband Rolf Hamburger by the Americans in Iran in April 1943. The Americans had informed the British that Hamburger had been actively engaged in espionage and sabotage. ‘On two occasions’, they said, ‘he was known to have bought information on railway and military installations – principally British – in Persia.’