Harry Potter and the Half-Blood Prince, which had been scheduled for release in December 2005, will instead go on sale July 16, publishers announced Tuesday.

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J.K. Rowling in 2003. (AP photo)

By Tuesday afternoon, the title held either first or second place on bestseller lists at Amazon's American and Canadian websites, as well as on the website of U.S. superstore Barnes & Noble. Soon after the publishers' announcements, online booksellers began offering the book for pre-order, often at a dramatic discount.

"Sales from the last Harry Potter book grossed as much as a major Hollywood movie in its first week of release," said Steve Riggio, CEO of Barnes & Noble, Inc. "We expect this next book in the series to make publishing history once again."

Raincoast Books, the Canadian publisher of the blockbuster series, called the upcoming release "the biggest publishing event of 2005."

"For J.K. Rowling's Canadian fans and for Canadian retailers, it looks like Christmas will be coming in July next year," said Raincoast CEO Allan MacDougall.

The British and American publishers of the series issued a joint statement: "J.K. Rowling has written a brilliant story that will dazzle her fans in a marvellous book that takes the series to yet greater heights" said Bloomsbury Publishing Plc in England and Scholastic Children's Books in the U.S.

Pregnant with her third child, the secretive Rowling had posted a message to her website in early December that led readers to believe the book's release was still far away.

"I have nothing noteworthy to report, because I have been spending nearly all my time sitting in front of my computer writing, rewriting and taking the occasional break to bang my head off the desk in frustration or else rub my hands together in fiendish glee (I think the latter has happened once)," she wrote.

However, on Monday, if visitors to the site answered three questions and solved three riddles in a special section, they were rewarded with Rowling's announcement about the release of Half-Blood Prince.

In the new posting, the 39-year-old British author said she had only "needed to tinker with the manuscript to my satisfaction and I am as happy as I have ever been with the end result. I only hope you feel it was worth the wait when you finally read it."

The previous instalment, the 870-page Harry Potter and the Order of the Phoenix, came out in the summer of 2003.

Though longer and much darker than the previous four novels – following Rowling's intention to reflect Harry growing up – readers snapped up the book. Phoenix sold 5 million copies within 24 hours of publication. According to Bloomsbury, the Harry Potter titles have sold 250 million copies worldwide, with the books being offered in 62 different languages.

Rowling had said that another of her beloved characters – one she refused to identify – would die in the sixth book. Although she assured that it wouldn't be Harry himself, she has also been mysterious about whether the titular hero would survive the seventh and final book.

The fourth film instalment of the series, Harry Potter and the Goblet of Fire, is also slated for a 2005 release. Directed by Mike Newell, the first British director to tackle the films, Goblet of Fire will introduce actor Ralph Fiennes as the evil Lord Voldemort.