Allen will play a Soviet spy in The Imitation Game
The plot revolves around developer Alan Turing, whose Enigma machine helped crack coded German military messages during the war.
Benedict, 37, is set to play the noted scientist – who committed suicide in 1954 – while Allen will take on the part of a Soviet spy who plots against him.
Keira, 28, is rumoured to be portraying a ‘woman from a very conservative background’.
Allen is currently filming the new series of Downton, which will see his character Tom Branson form a close relationship with his new governess.
Ex-chauffeur Tom was left devastated when wife Lady Sybil Branson – portrayed by Jessica Brown Findlay – died from eclampsia shortly after giving birth to their baby girl last year.
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