Author JK Rowling has condemned the reaction of "a bunch of racists" to a black award-winning actor playing the role of Hermione in Harry Potter and the Cursed Child. "With my experience of social media, I thought that idiots were going to idiot," Rowling said in an interview with the Observer.

"But what can you say? That's the way the world is. Noma was chosen because she was the best actress for the job. When John told me he'd cast her, I said, 'Oh, that's fabulous' because I'd seen her in a workshop and she was fabulous."

The 50-year-old British novelist has kept open the possibility of Hermione and other characters in the Harry Potter books could be played by a multi-racial cast.

"I had a bunch of racists telling me that because Hermione 'turned white' – that is, lost colour from her face after a shock – that she must be a white woman, which I have a great deal of difficulty with. But I decided not to get too agitated about it and simply state quite firmly that Hermione can be a black woman with my absolute blessing and enthusiasm."

Speaking of Hermione, JK Rowling said on the Pottermore website: "I saw Noma workshop the part and when John Tiffany [the director] told me he'd cast her, I was overjoyed. She gets Hermione inside out."

Olivier Award-winning Dumezweni hit back at critics. "It stems from ignorance. They don't want to be a part of the creative act. To say it's not as it was intended is so unimaginative," she told the Evening Standard.

"I don't think they understand how theatre works. We're here to heal you, make you smile and whisk you away. The only question we should ask is 'Are they good?' I've met great actors black and white and I've met bad actors black and white," she said.

Emma Watson, who played Hermione Granger in the Warner Brothers film franchise, gave her support in a tweet.

Another black actor, Cherrelle Skeete, plays Rose Granger-Weasley, daughter of Hermione and Harry's best friend Ron Weasley. Harry Potter and the Cursed Child, which officially opens on 30 July, has set a new West End record, selling 175,000 tickets in 24 hours.

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