The episode, Mike the Knight and the Christmas Castle, will transmit on CBeebies in the week running up to Christmas.
Armstrong, who can trace his ancestors back to William the Conqueror, also stars as the King in a feature length special of the hit animated show, Journey to Dragon Mountain, due for release next year.
Alexander commented: “To play the King in Mike the Knight is a real honour and it’s given me some kudos with my sons.”
Michael Carrington, VP content and head of production at Hit Entertainment, added: “Alexander is the perfect person to play the King; he has a fantastically regal air and brings the right balance of warmth, humour and authority to the role. We are delighted to have him on board.”
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