"Pottermore" is a mysterious website that has been taunting fans with words of "coming soon." Theories include a Harry Potter encyclopedia, e-book versions of the novels or an Internet-based game.
British newspapers received a leaked marketing memo indicating the site is a Web-based game. However, Rowling's spokeswoman Rebecca Salt said the December 2010 memo was "an out of date document that got sent out by mistake" and that true details of the project would be revealed Thursday.
Though "Pottermore" was trademarked in 2009 by Warner Bros., Salt said the site is not directly linked to the latest movie involving the boy wizard.
The final film in the Potter series, "Harry Potter and the Deathly Hallows, Part 2" premieres in London in July.
A leading Potter fan site, www.the-leaky-cauldron.org, said it had seen a preview of Pottermore and called it "breathtaking."
The Harry Potter books have sold about 450 million copies worldwide and spawned a hit movie franchise and a theme park. Forbes magazine has ranked Rowling as one of the richest women in Britain, with an estimated wealth of $1 billion.
The Associated Press contributed to this report.