Anthony Howell has been cast as Keel while Julian Wadham will play his sidekick Steed for the three volumes comprising 12 episodes. Only two of the season's 26 episodes exist in their entirety, with just 20 minutes of the first episode - Hot Snow - having survived as well.
Big Finish has signed a licensing deal with Studio Canal to produce full-cast audio dramas of a dozen of the missing episodes, including a full re-creation of Hot Snow.
Howell is probably best-known for his role as Sgt Paul Milner in 21 episodes of the ITV detective drama series Foyle's War, and he has recently filmed for the American series Dracula. His other work includes Dirk Gently - based on the books by Douglas Adams - and Dungeons and Dragons: The Book of Vile Darkness.
Wadham, meanwhile, can count Downton Abbey, Midsomer Murders, Middlemarch, and Blind Justice among his TV pedigree. He has also appeared in the films The Iron Lady (as Francis Pym), War Horse, Maurice, The English Patient, The Madness of King George, and Exorcist: The Beginning.
Volume 1 of The Avengers: The Lost Episodes will be released in January 2014, with Volumes 2 and 3 following in July 2014 and January 2015 respectively.