1. Ultra was the British code name for signals intelligence gained from Enigma, the electromechanical encryption system widely used by Germany's armed forces.
2. Sir Stewart Mitchell to Margaret Stewart, 10 August 1975, British Naval Intelligence Papers (MLBE), Churchill Archive, Cambridge, England.
3. Ibid.
4. The UK–US intelligence alliance in 1975: Economies, evaluations and explanations
5. H. Ranfurly,To War with Whitaker: The Wartime Diaries of the Countess of Ranfurly 1939-1945(London, 1995), 117.