In 1940, Matthews left the Labour Party, and during the war he was active in the National Union of Agricultural and Allied Workers. In 1943, he was appointed to the CPGB's Executive Committee, and in 1949, he became the party's Assistant General Secretary. He attended the 20th Congress of the Communist Party of the Soviet Union in 1956, and was acting editor of the party's "Daily Worker" newspaper during the Soviet invasion of Hungary. In 1947, he became the paper's Assistant Editor, moving to become Editor in 1949. In 1966, he gained agreement to rename the paper as the "Morning Star".