During World War II, the Security Service played a key role in combating enemy espionage, intercepting German communications and feeding misinformation back to Germany.
The Service achieved great success in uncovering enemy agents in Britain, some of whom were "turned" by the Service and became double agents who fed false information to the Germans concerning military strategy throughout the war. This was the famous "Double Cross" system, a highly effective deception that contributed to the success of the Allied Forces landing in Normandy on D-Day in June 1944 (see also Agent GARBO).
When captured German intelligence records were studied after 1945, it was found that almost all of the further 115 or so agents targeted against Britain during the course of the war had been successfully identified and caught. The only exception was an agent who committed suicide before capture.
For the first time, the Service also confronted the issue of loyalty of British Communists in sensitive government positions, in the certain knowledge that Party members were under instructions to "share" their information with Communist Party HQ. This was to have important repercussions following the war (see The Cold War).
The Service's success only came after an initial period of great confusion. The Service was inadequately prepared for the massive increase in work that came with the onset of war. It had far too few staff to deal with its new responsibilities. At the end of 1938, the Service had only 30 officers and another 103 secretaries and registry staff.
These problems meant that, when war was declared, a flood of reports, vetting requests and enquiries overwhelmed the Security Service. During the second quarter of 1940, the Service received an average of 8,200 vetting requests each week. The Service also had to contend with fears of a "Fifth Column" of Nazi sympathizers in Britain working to prepare the ground for a German invasion. This resulted in thousands of reports of suspected enemy activity, each of which had to be investigated.
The problem rapidly worsened with the introduction of internment (imprisonment without trial). Within the first six months of the war, 64,000 citizens of Germany, Austria and Italy resident in the UK had to undergo security interviews to confirm that they were "friendly aliens". In addition, suspected British Nazi sympathisers such as Sir Oswald Mosley were imprisoned to guard against the threat of domestic subversion.
In May 1940 the Service's chief Sir Vernon Kell was retired on the orders of the newly-appointed Prime Minister, Winston Churchill . He was replaced by Brigadier Oswald "Jasper" Harker, who was himself replaced by Sir David Petrie in April 1941.
Under Sir David, the Service underwent major reforms that greatly improved its ability to deal with the demands of wartime, and its major successes against German espionage followed. Some accounts of notable cases from this period are provided on pages linked below.
Collections of the surviving records from this period have been released to The National Archives, and further tranches of historical records continue to be released twice yearly (see The Security Service at The National Archives). More recent releases concerning the war have included personal files for German intelligence officers and agents, and files concerning "renegades" (British subjects in enemy or enemy-occupied territory who assisted the enemy in various ways, notably by broadcasting on behalf of Germany).
Included in the latter category were the personal files for the writer P.G. Wodehouse, and William Joyce (better known as Lord Haw-Haw). Many of the personal files contain details of the interrogations of German agents and officers carried out at Camp 020, the Service's wartime interrogation centre at Ham in Surrey.
There is also a range of associated material, including photographs, censored letters and recorded conversations. Also recently released are files on British Fascists, including Sir Oswald Mosely and Lady Diana Mosely.
The battle for the Rock of Gibraltar (1942-45)
How the Security Service defended Gibraltar from German and Spanish spies and saboteurs.
Agent GARBO (1941-44)
The story of Jean Pujol Garcia, codenamed GARBO, one of the most successful double agents in the history of espionage.
The fifth column case (1942-43)
How Eric Roberts prevented a network of Nazi sympathisers from becoming an effective fascist fifth column in Britain.
Hitler's last days (1945)
Eyewitness accounts of the last days of Adolf Hitler, gathered by Security Service interrogators following the end of the war.