He was Geneva's official interpreter for visiting Russians. It was long claimed that, writing as "C. Howard-Ellis", he wrote the text book for the League: "Origins, Structure, and Working of the League of Nations". However, it has been demonstrated that the real author was 'Dick' Ellis. Zilliacus maintained a correspondence with C. P. Scott of "The Manchester Guardian", which in 1935 helped generate popular support within Britain for sanctions against Italy should it attempt to conquer Ethiopia, an invasion which was launched later in the year. He wrote many articles and letters on international affairs on a "pro bono" basis, usually under pen names such as "Vigilantes". Zilliacus was a firm believer in the power of multinational organizations to prevent war, but he could not lead British foreign policy to work through the League. He worked diligently for the League of Nations until the Nazi invasion of Czechoslovakia, when he resigned from the Secretariat of the League of Nations. In World War II, Zilliacus worked for the Ministry of Information and joined the 1941 Committee. He was elected as MP for Gateshead in 1945 and became known as a left-wing critic of government foreign policy. Zilliacus was frequently accused of being a communist because he was sympathetic to Soviet policies and frequently contributed articles to liberal British publications, but he was not affiliated with the Communist Party of Great Britain. In 1949, he voted against joining NATO and remained an open critic of Foreign Secretary Ernest Bevin and his anti-Soviet policies. In 1949, he was expelled from the party, along with Leslie Solley. To compensate, he helped found the Labour Independent Group, although he would later leave the group when it supported Joseph Stalin over Josip Broz Tito. He sought re-election in the 1950 general election, but he lost his seat to Labour Party candidate Arthur Moody. Zilliacus was also sympathetic to Communist Yugoslavia. During the show-trial of Rudolf Slánský in Czechoslovakia in 1952, Slánský was forced to confess that he had given information to Zilliacus while "planning the restoration of capitalism in Czechoslovakia"; Zilliacus dismissed the accusation as "quite fantastic". In 1952, he was readmitted to the Labour Party, and he took Manchester Gorton district in the 1955 general election. He held the seat until his death, on 6 July 1967. He became a founder member of the Campaign for Nuclear Disarmament and in 1961 was suspended from the party for several months for writing an article for a Czech magazine.