In an older post (at this link) I had written about the loss of SS Thermopylae on 19 January 1942 during this operation of re-supply to Malta. I have now come across some information in British ULTRA intercepts that indicates that the Germans were fully aware of the importance of the operation in terms of supplies that were to be ferried to Malta, which would explain the heavy attacks on it. The intercepts are posted below. It would be interesting to see what the reaction to this was on the Royal Navy intelligence side – one would expect that they at least wondered if their codes had been broken.
MK 1834/AL 1678/MA 1383/WD 1004AIO
Germans appreciated on thirteenth that powerful reinforcements for Malta were being assembled in Alexandria. Convoy was expected to sail not before today fourteenth. Strong fighter protection to be given to a convoy of at least eight ships proceeding from Benghazi. Strong air ferrying detachments estimated at fifty aircraft to be flown from Benghazi. First flights already proceeded.
HMS NAIAD firing as she ploughed through heavy seas. IWM A9574
MK 1835 for Army Air only
With further reference to MK 1834 the German appreciation also included the following: the reinforcements include infantry, artillery, AX and “special formations”, searchlights, bombs, more than one hundred crates of engines, replacements, instruments, and “Diesel” engines which have been urgently requested by Malta.
In my view it is quite likely that this information came from the telegrams of Colonel Bonner-Fellers, the US Military Attache in Cairo, which were intercepted and read by the Italian secret service (see this older post). In any case, transmitting this information on over wireless was a grave breach of security on the German side, since it gave the game away to anyone listening in, that they had detailed knowledge of the load of this convoy.
I have come to the same conclusion as you with regard to the reports by Fellers, enabling the Axis forces to assemble forces to attack the Malta bound convoys. My sources to come to this conclusion were initially mostly secondary sources, such as Anthony Cave Brown’s, Bodyguard of Lies (pp. 101-2); Ronald Lewin’s Ultra Goes to War p. 175; Robert Ferris http://www.h-net.org/~diplo/reviews/ No. 199, pp. 2,4, 9; Christian Jennings, The Third Reich is Listening, p. 182. Another Fellers’s report, relating to the loss of Crete, is accessible from an American internet site; MA Cairo Egypt Report No. 1987, 09 Sep 1941, Airborne Invasion of Crete I.G. No. 9900, at http://www.allworldwars.com/Airborne-Invasion-of-Crete-1941.html
The original document, declassified on 19 Dec 2007, can be found here: https://cgsc.contentdm.oclc.org/digital/collection/p4013coll8/id/2507