JK Rowling has revealed she is preparing to write another children’s book.

Almost 20 years after the then penniless single mother sat down to write the Harry Potter series, she is returning to the genre that made her fortune.

Rowling, now 52 and worth close to £500 million, said: “I’ve just finished the fourth Galbraith novel, Lethal White, and I’m now writing the screenplay for Fantastic Beasts 3.

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“After that I’ll be writing another book for children. I’ve been playing with the story for about six years, so it’s about time I get it down on paper.”

The author did not reveal whether the as-yet-untitled book, the first for children she has written since Harry Potter and the Deathly Hallows, would be part of a series.

Writing in the Answers to Questions section of her personal website, the mother-of-three said there was a penalty to her fame – she can no longer write in public spaces.

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“I find human voices the most distracting when I’m working, although a background buzz, as in a cafe, is always comforting,” she said. “I used to love writing in cafes and gave it up reluctantly, but part of the point of being alone in a crowd was being happily anonymous and free to people-watch, and when you’re the one being watched, you become too self-conscious to work.”

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Rowling has an office in her Edinburgh garden with “a central room where I work, a kettle, a sink and a cupboard-sized bathroom”. She added: “The earlier in the day I start, the more productive I am.

“In the last year or two I’ve put in a couple of all-nighters on the screenplays for Fantastic Beasts, but otherwise I try and keep my writing to the daytime.”