Howard is best known for playing gentleman Ashley Wilkes in the 1939 cinema classic "Gone with the Wind." His other film credits include "Berkeley Square" and "The Scarlet Pimpernel."
The Daily Telegraph said Spanish author Jose Rey-Ximena claimed the 50-year-old actor's plane was shot down by the Luftwaffe in 1943 as he was returning to England after a trip through Spain and Portugal as part of a secret mission ordered by then-Prime Minister Winston Churchill.
Rey-Ximena said Howard's ex-girlfriend, Spanish actress Conchita Montenegro, who was married to a senior member of the Falangist party, helped secure a meeting between the actor and fascist dictator General Franco so Howard could convey a message in a bid to prevent Franco from teaming up with Adolf Hitler and Benito Mussolini.
"Thanks to him, at least in theory, Spain was persuaded to stay out of the war," the Telegraph quoted Rey-Ximena as saying.
Howard was killed before he could report back on his meeting with Franco, Rey-Ximena said, adding he learned of Howard's role in keeping Spain out of the war shortly before Montenegro's death last year at the age of 96.
"He has never been recognized as either a spy or as a hero," Rey-Ximena noted.