Thompson made the remarks at the premiere of her latest movie, Nanny McPhee and the Big Bang, and said the Henry Higgins role has yet to be cast, although Hugh Grant has been rumored as a possibility.
Thompson said she would "love (Grant) to do it", but added nothing had been confirmed yet.
When asked whether it was true that Mulligan had been cast in the musical movie, Thompson said: "I think Carey is set to play it, yes."
Mulligan, however, speaking to Collider, had this to say: "Oh, that's nice. Well, that's hugely flattering and that would be great. I really don't know, at this point, what's happening with that. That's lovely that Emma said that. That's really nice. But, I honestly don’t know."