Japanese codes were a longstanding interest of GC&CS
Japanese codes were broken as early as the 1920s, and Japan was a particular target for intelligence efforts due to the naval competition between the US, UK and Japan in the inter-war period. Japanese codes improved throughout the period of GC & CS operations but the codebreakers, with American help, were able to keep pace.
British codebreaking operations were initially carried out in Singapore, and then later Colombo, and Kilindini in Kenya. In addition increasing amounts of work were carried out at Bletchley Park itself, first on diplomatic and Naval traffic and later on Japanese Army and Airforce messages.
In addition to providing information about their own operations, Japanese messages also reported on German activities via their Military and Naval Attaches in Germany. This traffic provided significant advance warning of the German attack on the Soviet Union in 1941, and later provided a mass of vital information about the German defensive plans in France prior to the D-Day invasion.