[2][3] After the war, Liddell joined Scotland Yard where, in liaison with Special Branch and the Foreign Office, he was involved in breaking a spy ring based around the All Russian Cooperative Society in London.

[1][2][4] He transferred to MI5 with his team in October 1931, where he became an expert on Soviet subversive activities within the UK and recruited agents, including his private secretary Dick White, and future head of B5(b) Maxwell Knight, in preparation for possible war with Germany.

[3] Following the outbreak of the Second World War, Prime Minister Winston Churchill sacked Director-General of MI5 Vernon Kell and in June 1940 Liddell was promoted to Director of B Division in charge of counter-espionage, where he appointed Dick White and Anthony Blunt to senior posts.

Shortly after the new appointment, he was informed by Maxwell Knight of a suspected German spy-ring based around the Right Club of Archibald Ramsay and involving American cipher clerk Tyler Kent.

[1][2][3] Liddell was expected to succeed David Petrie as Director General of MI5, but was passed over when Home Secretary Herbert Morrison was informed by Ellen Wilkinson of rumours that he might be a double agent and was instead appointed Deputy-Director-General under Percy Sillitoe.

He was also a known associate of Kim Philby and Anthony Blunt, other members of the Cambridge Five spy ring; and in 1953, following an MI5 internal investigation, he took early retirement and went to work as a security adviser to the Atomic Energy Authority.

[9] Rupert Allason (2018) (writing as "Nigel West")[10] made no suggestion that Liddell had been responsible for any treachery, and in fact, claimed that he "was betrayed by Burgess, Blunt, and Philby".