More than 150 files created during the period of the Great War, but often extending beyond it, have been digitised by the National Archives, and include interrogation reports, letters, postcards and photos relating to individuals and groups under surveillance.
Part of a wider security service personal file series held by the National Archives, based in Kew, south-west London, they form part of its First World War 100 programme of digitised releases and events to mark the centenary of the war.
The files contain details of spies around the world, including famous people like British nurse Edith Cavell, who saved soldiers in German-occupied Belgium; Mata Hari, the notorious female spy and entertainer; Arthur Ransome, author of Swallows And Amazons, and American poet and author Ezra Pound.
They also contain intelligence reports and surveillance of organisations such as the Bolshevik Party, British Communist Party, and the Boy Scout Association, as well as files on political figures from known fascists to communists and Russian leaders such as Leon Trotsky and Vladimir Lenin.
National Archives records specialist Dr Stephen Twigge said: "The files in the National Archives' collection reveal the importance of the security service in safeguarding the nation during the First World War.
"Now that we have made the files available online as part of our First World War 100 programme, people across the globe can discover the secret history behind the war for themselves."
See the records at www.nationalarchives.gov.uk