Harold Russel "Kim" Philby worked with Graham Greene in MI6 [SIS].
Graham Greene wrote The Third Man, published in 1950. In order to gather information for the novel he visited Vienna in February 1948, where he contacted, via Elizabeth Susan Douglas-Scott-Montagu [obituary here], Peter Smolka [latter Smollett], a good friend of Philby, agent of the British intelligence as Philby and a soviet mole also.
I just returned from Vienna, were I visited The Third Man Museum [see the link here for the site] and brought from there a booklet concerning Post War Vienna. The magnificent museum is dedicated to both Greene's novel and Orson Welles film, an initiative of Gerhard Strassgschwandtner and Karin Höfler, without sponsors or subsidies.
There I found an article by John Beauclerk where he states that Greene's source about the sewer of Vienna, where the plot has one of its more dramatic moments, was not, as Greene's try to convince readers of the novel, his father, Colonel Charles Frederick Aubrey de Vere Beauclerk [later the 13th Duke of St. Albans, married with Suzanne Fesq, obituary here], but probably "Kim" Philby himself, who had been in the city during 1934 working with the communists.
Reason for that? According with Jon Beauclerk, the purpose was to hide Philpy's cover as a secret agent. Moreover he adds: «the film is Greene's way of dealing with the moral ambiguity of his relation with Kim Philby».