Here is the official press release for Keeley Hawes.
Keeley Hawes (Line of Duty, Upstairs Downstairs) will join the cast of Doctor Who, starring opposite the Doctor. Set to return to BBC AMERICA this year, Hawes will play Ms. Delphox, a powerful out of this world character with a dark secret. Travelling across space and time, the Twelfth Doctor (Peter Capaldi) and his companion, Clara Oswald (Jenna Coleman), will come face to face with the mysterious Ms. Delphox when they arrive on a strange and puzzling planet.
Written by Steve Thompson (Sherlock) the episode will be directed by Douglas Mackinnon (Line of Duty, Doctor Who)
Commenting on her role, Hawes, said: “I am delighted to join Doctor Who and to be working with this incredible team. Ms Delphox is a great character and someone I've had a lot of fun playing.”
While lead writer and executive producer, Steven Moffat, added: "Anyone watching Jed Mercurio's amazing Line of Duty will know that Keeley Hawes is having one hell of year. And now it's about to get even better as she achieves the greatest villainy yet attempted on Doctor Who: she plays a banker."
Doctor Who is a BBC Wales production for BBC One. Filming is currently underway for the new series, which will introduce Peter Capaldi as the next Doctor for the first time to millions of viewers.