LOS ANGELES — The release date for the sixth Harry Potter movie, "Harry Potter and the Half-Blood Prince," was pushed back on Thursday to July 2009 from its original slot in November 2008, movie studio Warner Bros. said.

The eight-month change in the opening date to July 17, 2009, is expected to disappoint millions of Harry Potter fans around the world who have already been waiting more than a year to watch the next big-screen installment in the international literary and film phenomenon.

Trailers for "Half-Blood Prince" began running last week in advance of the expected November movie release worldwide.

Warner Bros., a unit of Time Warner Inc, said it had decided to make the change to bolster its release schedule for the lucrative summer moviegoing season -- a period that can account for as much as 40 percent of Hollywood's annual box office receipts.

Warner Bros. President Alan Horn said the studio was also still feeling repercussions from the three-month Hollywood screenwriters strike that ended in February but has "impacted the readiness of scripts for other films."

"We know the summer season is an ideal window for a family ... release, as proven by the success of our last Harry Potter film, which is the second-highest grossing film in the franchise, behind only the first installment," Horn said.

The studio said the change of date would not alter production plans for the final, two-part Potter movie adventure, "Harry Potter and the Deathly Hallows." The release date for part one is tentatively set for November 2010.

"Deathly Hallows," the seventh and last story from British author J.K. Rowling about the boy wizard and his friends at Hogwarts School, was published in July 2007. The series has sold an estimated 400 million copies, and the books have been translated into more than 60 languages.