In 1940 he acted in "When Knights Were Bold", an Army production,
after which his photograph appeared in the paper with the reference to
how much he looked like General Montgomery. Some four years later this
had far-reaching results when, in May 1944, he was approached by actor
David Niven, then a Colonel in the Army Kinematograph Section, and
asked to participate in the making of Army movies. However, it turned
out that David Niven had something other than movies in mind.
Sent to London, he was told he was wanted to play the double of
"Monty", as his public presentation of "Monty" would allow Montgomery
to be somewhere else, so confusing the Germans. To be able to do this
he had to learn his gestures, mannerisms, gait and voice. Most
difficult of all, Clifton James had to give up smoking. Having lost
his right-hand middle finger in the First World War, a realistic
replacement was made. Even his wife had to be deceived and was both
kept in the dark and sent back to Leicester.
Once he was trained, his trip as "Monty" was to Gibraltar and from
there to Algiers. "Monty's" presence even succeeded in confusing the
Germans in regards to the invasion plans.
 

Source: Leo van de Pas