Christine Chilver in her glamorous ball gown looking like a 1940s film star. (Courtesy of Paul Tonge)

Christine relaxing in a garden. (Courtesy of Paul Tonge)

Portrait of Christine. (Courtesy of Paul Tonge)

The ‘Keep mum – she’s not so dumb’ campaign was issued in 1942 to caution against careless talk. It was intended to remind soldiers of all ranks that when in the company of a beautiful woman, beauty may conceal brains, and that women could act as spies as well. (Courtesy of the National Archives)

Flying Officer Cyril Miller, security officer at ‘Bayswater’ (MI5’s interrogation centre), who oversaw Fifi’s work. (Courtesy of the National Archives)

The Official Secrets Act signed by Christine on 31 January 1943. (Courtesy of the National Archives)

Rosemary ‘Pat’ Say, who was interned in France with Christine, but escaped back to England and worked in SOE’s F Section. (Courtesy of the National Archives)

Gunnar Tingulstad, provoked by Fifi in Cardiff on 1 January 1943. (Courtesy of the National Archives)

Tobias Skog, alias Peter Larson, provoked by Fifi in Wolverhampton in December 1942. (Courtesy of the National Archives)

Euan Butler, alias Captain Beresford, provoked by Fifi in Bristol in June 1943. (Courtesy of the National Archives)

Cecile ‘Pearl’ Witherington, code-named Pearl Wimsey, whom Fifi attempted to provoke at Birmingham in August 1943. (Courtesy of the National Archives)

German telegram to Friedl Gartner, deception agent Gelatine, informing her that her cover address in Lisbon was blown. (Courtesy of the National Archives)

Natalie, or Lily, Sergueiew, alias Treasure, double-crossed the German secret service by sending them misleading messages created by MI5. (Courtesy of the National Archives)

Lily’s identity card. (Courtesy of the National Archives)

Lily with Emil Kliemann in Lisbon, March 1944. Kliemann was Lily’s case officer for the Abwehr, German military intelligence. MI5 had sent her to reassure him that she was still his agent. (Courtesy of the National Archives)

Examples of the code Lily used to encrypt her messages to the Germans. (Courtesy of the National Archives)

One of Elvira Chaudoir’s letters with the secret ink exposed. (Courtesy of the National Archives)