Alexander, (Conel) Hugh (O'Donel) (1909–74), chess master and code-breaker, was born 19 April 1909 in Cork, eldest child of Conel William Long Alexander (qv), professor of engineering at UCC, and Hilda Barbara Alexander (née Bennett) of Birmingham. After his father's death the family moved to Birmingham, where he attended King Edward's School. Entering King's College, Cambridge (1928), he monopolised the university chess championship and represented the university (1928–31) in the match against Oxford. Graduating with first-class honours, he taught mathematics at Winchester College (1932–8). He joined the John Lewis partnership as head of personnel in 1938, a position he held till the outbreak of war and for a brief spell after the war (1945–6). Meanwhile, he secured a regular place on the British international chess team (1931–58) and represented Britain with distinction in the Olympiads of the 1930s and 1950s. Individual success came in 1938 with his first victory in the British championship, a victory he repeated in 1956. In 1940 he joined the government code and cipher school, Bletchley Park, as deputy (later successor) to Alan Turing in charge of Hut 8, decrypting the German naval ‘Enigma’ – work for which he was awarded an OBE (1946). After the war he took charge of research and development at Government Communications Headquarters (GCHQ), Cheltenham, and joined the teaching staff at the imperial Defence College in 1956. The continued success of his work earned him a CBE (1955) and a CMG (1970). He declined an opportunity to become bursar of King's College, Cambridge, and retired in 1971.
Although the war had interrupted his chess career, he marked his return with a victory over the Russian grand master Mikhail Botvinnik (1946). He was first at Hastings (1946–7) and joint first with David Bronstein in 1953 in a match that made chess front-page news in Britain. Always the consummate strategist, he had many fine results against some of the world's leading players. Owing to the demands of his profession he gave up competitive chess completely in 1963, but acted as non-playing captain of the British team (1964–72). After retiring he devoted himself to writing about the game, producing several regular newspaper columns and a number of popular books. He played in Ireland only once, representing England in 1957; he also turned down an invitation to play for Ireland in 1935. He died at his home 15 February 1974, after a recurring illness. An international tournament, the Alexander Memorial, was held in 1975 to honour his services to chess.
He married (1934) Enid Constance Crichton, daughter of Ronald Neate, an Australian sea captain; they had two sons. After the war they separated, Enid returning to Australia with their younger son, Patrick. The older, Sir Michael O'Donel Bjarne Alexander (1936–2002), born 19 June 1936, spent his early years in Ireland and was educated at Foyle College, Derry; St Paul's, London; and King's College, Cambridge. An Olympic fencer and capable chess player, he joined the British diplomatic service, acting as diplomatic secretary to Margaret Thatcher (1979–82) and ambassador to Vienna (1982–6) and NATO (1986–91); he was appointed CMG (1982), KCMG (1988), and GCMG (1992). On retiring (1992) he directed banking enterprises in eastern Europe and became (1993) chairman of the Royal United Services Institute for Defence Studies. He died of cancer on 1 June 2002, and was survived by his wife Traute (née Krohn), whom he married in 1960, and by their two sons and a daughter.