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Having announced a new website (www.pottermore.com), the British author has kept its contents a closely guarded secret, giving previews to just a handful of the biggest fan sites who have been sworn to silence.
All will be revealed on Thursday, with a countdown clock linked to the website indicating that it will go live at noon London time.
Not a new book
"The owls are gathering ... Find out why soon" is all the link says.
What many Potter fans would like most would be an eighth novel to follow on from the seventh and "final" instalment of Harry Potter and the Deathly Hallows published in 2007.
But Rowling's publicist has already made clear that it would not involve a new book.
The seven-novel series has sold more than 400 million copies worldwide, and, along with the blockbuster movie franchise, turned Rowling into the world's wealthiest writer.
Other options on the website include a social networking fan site, an online game or the launch of the novels as e-books.
The announcement comes just a few weeks ahead of the theatrical release on July 15 of the eighth and final Harry Potter film which will be shown in 3D. The seven movies released so far have grossed $6.4bn in ticket sales.
