The new additions complete the powerhouse European cast of Emily Blunt, Rupert Friend, Miranda Richardson, Jim Broadbent, and Mark Strong.
French Canadian Jean-Marc Vallee is directing the drama about the early years of Queen Victoria, Britain's longest serving monarch, and her romance with Prince Albert.
The film is booked in for a 10-week shoot at Shepperton Studios and is based on a screenplay by Julian Fellowes.
Bettany, whose credits include Master And Commander, The Da Vinci Code and the upcoming Inkheart, will play Queen Victoria's closest advisor Lord Melbourne.
Kretschmann, who starred in King Kong, Downfall (Der Untergang) and The Pianist, will play the Belgian King Leopold. Kretschmann's upcoming credits include Valkyrie and Wanted.
Jesper Christensen, Michael Maloney, Julian Glover and Harriet Walter round out the cast.
The GK Films production is being produced by King and Scorsese, with Tim Headington and Sarah Ferguson also attached. Denis O'Sullivan is overseeing the project for GK Films.