JK Rowling has succeeded in a US legal bid to block the publication of an unauthorised encyclopedia of the world created in her best-selling series of Harry Potter books.

A judge ruled today that the author proved that Steven Vander Ark’s ‘Harry Potter Lexicon’ would cause her irreparable harm as a writer.

Rowling and Warner Bros, the Hollywood Studio behind the film adaptations of her books, sued US independent publisher RDR Books last year.

In emotional appearances at Manhattan Federal Court last spring, the author had warned that the floodgates would open on unauthorised versions, and spinoffs, of her books if a judge allowed publication.

She had to ask for a glass of water to regain her composure when asked to describe what her seven-book series meant to her. “I really don’t want to cry because I’m British. It means setting aside my children and everything,” she said.

“These characters meant so much to me, and continue to mean so much to me, over such a long period of time. It’s very difficult for someone who is not a writer to understand. The closest I can come is to say to someone, ‘How do you feel about your child?’.”

Vander Ark had described the book as a modest but extensive guide for fans like himself, and not an attempt at serious scholarship. US copyright law allows teachers, academics, journalists and critics to use excerpts of an author’s work, but on a limited basis.

The unauthorised encyclopedia was compiled through a website, the Harry Potter Lexicon, which was founded by Ark.

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