The 90-minute BBC Two programme is based on the West End and Broadway play King Charles III, written by Doctor Foster scriptwriter Mike Bartlett.
Charlotte said (via The Independent): "It's such a unique project, to be both modern and rich in verse and to play someone who is real but yet totally re-imagined for this story.
"Kate Middleton is a really interesting woman, particularly within the context of this play, and it is a challenge I am really looking forward to."
King Charles III imagines a world where Prince Charles has taken the throne, but chaos ensues across the country when he refuses to sign a controversial bill.
Writer Mike Bartlett said: "I couldn't be more excited that King Charles III is now fully, brilliantly cast and about to begin shooting. We have an exceptional team, both in front of and behind the camera, and I'm looking forward to bringing this very contemporary royal tragedy to a national television audience."
Prince Charles – or King Charles, as he is here – will be played by Tim Pigott-Smith, who performed the role on stage.
Other castmembers include Green Wing actor Oliver Chris as Prince William and The Iron Lady's Richard Goulding as Prince Harry.
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