LONDON (Reuters) - Author J.K. Rowling is working on a Harry Potter encyclopedia, and will donate the royalties to charity, she has said on her website.

Four years ago the British writer took an independent U.S. publisher to court to stop its plans for a reference book on the world of the fictional boy wizard, saying it constituted a "rip off" of her work.

On her re-vamped website, Rowling addressed the question of a Harry Potter encyclopedia in a "frequently asked questions and rumors" link.

"For a long time I have been promising an encyclopedia of Harry's world, and I have started work on this now," said the 46-year-old, whose announcements related to Harry Potter are closely followed by millions of fans.

"Some of it forms the new content in Pottermore."

Pottermore, a free website allowing visitors to navigate the stories and discover new content not in the original books, is one of several Potter spin-offs following the completion of the seven-novel series.

The novels have sold more than 400 million copies worldwide and spawned a hit eight-movie film franchise that have helped turn Rowling into the world's most successful author.

"It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity," Rowling added.

She again addressed the most burning question for die-hard followers of Potter and his pals atHogwarts School of Witchcraft and Wizardry -- would there ever be another Harry Potter story?

"I have always refused to say 'never' to this question, because I think it would be foolish to rule out something I might want to do in a few years' time," she wrote.

"However, I have no immediate plans to write another Harry Potter novel, and I do think that I have rounded off Harry's story in the seven published books."

Earlier this month, Rowling announced the title of her first book for adults would be "The Casual Vacancy". It goes on sale on September 27.

(Reporting by Mike Collett-White, editing by Paul Casciato)