Harry Potter author JK Rowling has won her claim that a fan violated her copyright with his plans to publish a Potter encyclopaedia.

Judge Robert Patterson ruled on Monday that Ms Rowling had proven that Steven Vander Ark's Harry Potter Lexicon would cause her irreparable harm as a writer.

Ms Rowling sued Michigan-based RDR Books last year to stop publication of the lexicon. Vander Ark runs the popular Harry Potter Lexicon website.

Victory: A judge today ruled that a Harry Potter encyclopaedia written by Steven Vander Ark (r) violated the copyright of JK Rowling (l)

In April, Judge Patterson told the U.S. District Court in New York that he had found some of the multi-millionaire author's Potter storylines confusing.

Reading half of the first book - Harry Potter and the Philosopher's Stone - to his grandchildren, he said he found Ms Rowling's 'magical world' hard to follow, filled with strange names and words that would be gibberish in any other context.

Earlier in the trial, the world-famous writer showed a flash of temper when she claimed that the unauthorised lexicon could lead to countless other rip-offs of authors' works.

'I didn't fly here because I was worried about losing sales,' she said, her voice rising.

'I'm here because I passionately believe this is about authors' rights to protect their creation.

'If this book is published, I believe the floodgates will open. Are we the owners of our own work?'

Ms Rowling plans to write her own companion guide and donate the proceeds to charity.