MI8

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MI8, or Military Intelligence Section 8, was Britain’s War Office unit for signals intelligence. Created in 1914, it had four parts: MI8a (wireless policy), MI8b (cables), MI8c (distribution of censored intelligence), and MI8d (liaison with cable companies). In World War I, MI8 officers worked at cable hubs in Cornwall and Ireland, and the work continued until 1917 when the Admiralty took over.

In World War II, MI8 ran the War Office’s large signals network, known as Y Group, and briefly ran the Radio Security Service (RSS) from late 1939 to mid-1941 before it was handed to MI6. RSS began as a way to locate illicit wireless transmissions inside Britain and grew with help from MI5 and the General Post Office. It recruited thousands of amateur radio operators as volunteers to intercept signals and locate transmitters.

For a time RSS looked for German agents in Britain, but by 1940 it found few such transmissions. The service moved to Arkley near Barnet to handle the workload, and tens of thousands of intercepted messages were sent to Bletchley Park for codebreaking. In 1941 control of RSS passed to MI6, with a central station at Hanslope Park. By decoding Abwehr traffic, Bletchley Park could read German plans. At its peak, RSS had about 1,500 personnel, many of them amateur operators. After the war RSS HQ moved to Eastcote and joined GCHQ.

MI8 and RSS played key roles in Allied signals intelligence, helping reveal enemy plans and supporting victory.