MI6 has its origins in the Secret Service Bureau, which was established in 1909 under Mansfield Cumming to provide intelligence on the German naval build-up and an early warning of invasion. During the First World War the service formed close liaisons with the intelligence agencies of the other Allied powers, including the various competing services of Tsarist Russia. It is not clear how effective these liaisons were against the common enemy, but the Russians valued Cumming sufficiently to award him an honour.

In the chaos following the 1917 Bolshevik Revolution, a small number of people whom MI6, the Foreign Office, the Admiralty and the War Office had sent to support the ailing Russian war effort became Britain’s – indeed, the world’s – only sources of information on events inside Russia. When Lenin’s terror set in, a handful of them remained in Russia to spy for Cumming. For the next few years most of what little information there was about the Soviet government’s secret actions and intentions came from them. This book is about what they achieved.

Giles Milton has read widely among published and unpublished sources, making particularly good use of the 751 files and volumes from Indian Political Intelligence released in 1997. He has also trawled the National Archives, finding gems among declassified MI5 files, and has mined what he can from Keith Jeffery’s authorised