The author had previously spoken of plans to create a "definitive guide" which would detail creatures, characters, places and other important parts of the hugely popular seven-book series.
© PA Images / Dominic Lipinski/PA Wire
After the planned launch of interactive website Pottermore - which also provides previously-unknown details about elements of the story - the future of the book had been left in doubt.
However, on her relaunched official website, Rowling informed fans that work on the charity book is now under way.
"For a long time I have been promising an encyclopaedia of Harry's world, and I have started work on this now - some of it forms the new content in Pottermore," she explained. "It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity."
Addressing the possibility of another novel in the Harry Potter series, the 46-year-old reiterated that she has "always refused to say 'never' to this question".
"I think it would be foolish to rule out something I might want to do in a few years' time. 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," she said.
Meanwhile, commenting that she has not managed to "tweet very often" on her Twitter handle @jk_rowling, the author quipped: "However these fake JK Rowlings do seem to lead very exciting lives..."
It was announced yesterday that Rowling's new adult novel The Curious Vacancy will be published on September 27.
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