Diplomatic messages sent by both Allies and Enemies were also broken by GC&CS
Deciphering diplomatic cables had been the main task of GC & CS between the wars, and this work continued after the move to Bletchley Park. Diplomatic Section was moved in late 1939 into Elmer’s School, a former school building adjacent to the park. Later, in 1942 Diplomatic and Commercial work returned to London and was housed at 6-9 Berkeley Street in Mayfair. It was at this time that Alistair Denniston took over the section after he stepped down as head of Bletchley Park.
Some diplomatic communications were sent from Embassies based in London by radio and these were intercepted by a Metropolitan Police intercept station based in Denmark Hill, south London, and sent to the codebreakers. But most were sent in enciphered telegrams which travelled via the international telephone and telegraph cables. Many international telephone and telegraph lines were controlled by the British company Cable and Wireless and two US companies operating in the UK. The Germans, Japanese and the Italians still used them believing that because their telegrams were all enciphered they were unreadable. Some cables were cut, but key ones remained, so for example the Japanese Ambassador’s reports about the Normandy defences, and those of his Army and Navy colleagues, all went by cable via Malta, where they were printed out and sent back to Bletchley. Similarly large numbers of telegrams for and from Japanese and German Embassies in the Americas went through Bermuda where we also printed them out and sent them back to the UK.