The suit is prompted by the imminent publication of a book based on the website, The Harry Potter Lexicon. Rowling and the producers of the Harry Potter films Warner Bros are seeking an injunction to stop the publication of the book, claiming copyright infringement.
The case, which will be heard in New York on 14th April, centres on what exactly constitutes copyright infringement and whether or not the Potter copyright owners' seeming acceptance of the Lexicon as a website affects their ability to stop it becoming a book.
Potter fan Steve Vander Ark began putting the online Lexicon together in 1999, cataloguing the peculiarities of the world Rowling had created for her bespectacled hero-conjurer. Fans have contributed to the site over the years and it is now widely accepted as the definitive authority on Potter's world.
Small Michigan publisher RDR Books has been working with Vander Ark on a book version of the Lexicon but Rowling and Warner claim that this infringes their copyright.
"The people, places, terms and images in the Harry Potter books … are Ms Rowling's original creative work," says the lawsuit.
"I am deeply troubled by the portrayal of my efforts to protect and preserve the copyrights I have been granted in the Harry Potter books," said Rowling in court papers. "If RDR's position is accepted, it will undoubtedly have a significant, negative impact on the freedoms enjoyed by genuine fans on the internet."
RDR claims that its publication of the book would be within the fair use exemptions of copyright legislation.
"[Rowling] appears to claim a monopoly on the right to publish literary reference guides, and other non-academic research, relating to her own fiction," said a statement from RDR. "This is a right no court has ever recognised."
"It has little to recommend it," said RDR. "If accepted, it would dramatically extend the reach of copyright protection, and eliminate an entire genre of literary supplements: third party reference guides to fiction, which for centuries have helped readers better access, understand and enjoy literary works."
The suit is based on US copyright law. John Mackenzie, a litigation specialist with Pinsent Masons, the law firm behind OUT-LAW.COM, said that if it were a case under UK copyright law, it may not succeed.
"If what this book is doing is identifying individual aspects of a particular thing such as characters, referring to those individual parts is probably not copyright infringement," said Mackenzie.
"Vander Ark says he is not copying the books, that he is making general reference to the original work. If that is so, as he builds it up his reference work becomes a creation in its own right, not a copy," said Mackenzie. "It is then a derivative work, which is quite distinct from the original, and a derivative work is allowed. Of course, if all he has done is copy chunks of the Harry Potter books and put them into a different order, that would be different. The court could ask 'Is he taking advantage of the skill, time and effort that JK Rowling put into writing the books?'"
The case is complicated by the fact that Vander Ark claims that Warner Brothers, Rowling and her book publishers all knew of, expressed approval of and accepted the Lexicon as an online work.
RDR claims that Warner Brothers used the timeline created for the website on its DVDs of the Harry Potter films, even including typographical errors from the website timelines. Warner Bros' and Rowling's lawsuit calls this "a complete fabrication".
Vander Ark told The Guardian newspaper that the film producers had used and approved of his work while making the films.
"Warner Brothers flew me over to the UK to meet the stars and the crew for the opening of Harry Potter and the Order of the Phoenix," he told the paper. "The producer, David Heyman, told me they used the Lexicon on set every day. I was also taken to Electronic Arts, the company that was making the video game of the film, and they had printouts of the Lexicon taped all over their walls."
The site has also been awarded one of Rowling's "fan site awards".
Mackenzie said that if the case were heard in the UK it could turn on the issue of the apparent approval of the various copyright owners in the Lexicon up until now.
"There is a very interesting question of waiver or acquiescence," he said. "If he manages to prove that Rowling has known of this work, has used it, that it has been used on the film set and that everybody has known of it, the right to sue may be lost."
The lawsuit claims that the very fact of taking material from a website platform to a written one changes the nature of any infringement. "There is a big difference between the innumerable Harry Potter fan sites' latitude to discuss the Harry Potter works in the context of free, ephemeral websites and unilaterally repackaging those sites for sale in order to cash in monetarily on Ms Rowling's creative works in contravention of her wishes and rights," said the suit.
Mackenzie said that under UK law the copyright status of the work would not be affected either by the Lexicon's move from the web to print or from free to paid-for.
Editor's note, 20/03/2008: For avoidance of doubt, we have not seen the Lexicon. We have changed a comment that appeared in the original version of this story which may have been interpreted as suggesting otherwise. We apologise for any confusion caused.