"For a long time I have been promising an encyclopedia of Harry’s world, and I have started work on this now – some of it forms the new content in Pottermore. It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity."

She didn't reveal any more details about the content of the book or when it might be finished. Rowling has written three other Potter-related books: Fantastic Beasts and Where to Find Them, Quidditch Through the Ages and The Tales of Beedle the Bard.

Rowling reiterated that she has no "immediate plans” to write a new Harry Potter adventure, but she added, "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."

The news comes at the same time Pottermore, the online site for all things Harry Potter, finally opened to the public on Saturday. The site, which had been in beta testing since late last year, offers interactive games, behind-the-scenes information and bonus content.

One of the cooler aspects of Pottermore is the ability to put on the famous sorting hat to get placed into one of the four Hogwarts Houses (confession: I ended up in Slytherin).

At the moment, the interactive games involve the first book, Harry Potter and the Sorcerer's Stone, but the plan is to add games relating to the subsequent volumes over time. Pottermore users can also purchase the newly released ebook versions of all seven Harry Potter books on the website.

Pottermore can be found here.