Detail from Cupid Complaining to Venus by Lucas Cranach. Photograph: AP
It is possible that the Cranach, painted in the 1530s, was wrongfully taken from a Jewish collector, and the National Gallery is appealing for anyone with evidence to step forward. If proof were available the Cranach would have to be returned to the heirs of its legal owners.
The painting, removed from the Munich flat during the war to protect it from air raids, had ended up in a depot in southern Germany. Hartwell was given the painting by the local US commander as a thank-you for writing a positive piece about the local military administration.
Dr Birgit Schwartz, a Vienna-based art historian, has been researching Hitler's art using his personal library of 1,200 books, now housed in the National Archives in Washington. Among them, completely unremarked upon until Dr Schwartz happened upon it recently, was a parchment-bound album that a pencilled note identified as full of photos of Hitler's own paintings.
The dictator's love of art is well-known: apart from studying to be a painter, from 1938 he took a great personal interest in amassing works for a grandiose art gallery in Linz, Austria, much of it stolen or confiscated from Jewish collectors. The Munich flat was never photographed for security reasons and so little has been known about its contents. Two lists of artworks in the flat were made, one by a friend of Hitler and one shortly after the war, but because Cranach painted a number of versions of the same subject the National Gallery picture was not linked to Hitler's picture until the photograph emerged.
According to an article by Dr Schwartz in the Frankfurter Allgemeine Zeitung, the album reveals that the Cranach was something of an anomaly for Hitler, as his taste leaned heavily towards minor German masters of the 19th century, including Carl Spitzweg, Heinrich Bürkel and Rudolf Epp.
One clue is provided as to why Hitler might have owned Cupid Complaining to Venus: in 1939 a British journalist, Ward Price, noted that Hitler had a Cranach in the Munich flat, and that it had recently been given to him as a 50th birthday present by the regional commander of Thuringia, Fritz Sauckel.
According to Dr Susan Foister, the National Gallery's director of collections, the painting "shows Cranach at his best, when he was working at the Saxon court. There's the beautiful, almost naked woman with her wonderful plumed hat, the fabulous landscape in the background and the delightful little figure of Cupid in the foreground."