JK Rowling has warned that “the floodgates will open” on unauthorised versions of her books if a judge allows the publication of a Harry Potter encyclopaedia.

The British author was testifying at the Manhattan federal court, which she is asking to block publication of The Harry Potter Lexicon, a guide to the characters, places and spells in her novels.

Returning to the witness stand on the last day of a three day trial, Ms Rowling said: “I believe the floodgates will open. Are we the owners of our own work?”

The offending reference guide was written by Steven Vander Ark, a 50-year-old former middle school librarian who compiled the material from a website by the same name that he had been operating for years.

RDR Books, the small publisher that persuaded Mr Vander Ark to put material from the website into print, has argued that it is no different than any other reference guide and should be allowed to go to press without interference.

Ms Rowling said she was “vehemently anti-censorship" and generally supportive of the right of other authors to write books about her novels. But she said Mr Vander Ark had “plundered her prose and merely reprinted it in an A-to-Z format”.

In a statement, the multi-millionaire author said that the book had overstepped a boundary.

“Authors have a right to protect their works from misuse. Do I have fewer rights because many people read my books? If this book is published, it will open the floodgates for anyone to lift an author’s work and present it as their own.”

Robert Patterson Jr, the US district judge, asked Rowling whether she thought anyone would read the lexicon and its list of facts about the wizarding world for entertainment value.

“Honestly, no,” Rowling said. The good parts, she said, have simply been lifted from her own work.

“I think there are funny things in there, and I wrote them,” she said.

A decision in the case is not expected soon. It will be weeks before lawyers finish filing legal documents, and possibly longer before a verdict is reached. Judge Patterson is deciding the case, rather than a jury.

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