The actor will play the Prince of Wales in the controversial fifth series of the Netflix hit.
Imelda Staunton, Elizabeth Debicki and Dominic West are all joining the award-winning drama for its fifth season.
Elizabeth Debicki and Dominic West are taking over from Emma Corrin and Josh O’Connor in the Netflix drama.
The British star appeared on Jimmy Fallon’s US chat show a week after she was spotted in Rome with the married actor.
The actor was seen cosying up to co-star Lily James during a lunch date in Rome over the weekend.
The Good Morning Britain star's husband has been in intensive care for the last six months.
"I did slightly leap in the air with joy," the actor admitted to Adil Ray and Kate Garraway.
"I've started singing every song whenever a line makes reference to one."
'Even after a Golden Globe I’m not going to be on parity.'
The drama starts filming next month.