The release date for Harry Potter and the Half-Blood Prince has been pushed back by eight months, to July 2009.
The sixth instalment of the teenage wizard's adventures was supposed to have its Royal premiere on 17 November.
Alan Horn, president of studio Warner Bros, said the decision was taken to guarantee the studio a major summer blockbuster in 2009.
He also blamed the Hollywood writers' strike, which hit the film industry hard earlier this year.
Mr Horn said the strike, which ended in February, had "impacted the readiness of scripts for other films."
Fantasy books
He said: "The picture is completely, absolutely, 100% on schedule, on time. There were no delays.
"I've seen the movie. It is fabulous. We would have been perfectly able to have it out in November."
The move will mean a two-year delay between the film adaptations of books five and six in JK Rowling's fantasy series.
But it will shorten fans' wait between Half-Blood Prince and the final two instalments of Harry Potter and the Deathly Hallows, which are being shot simultaneously next year.
It is thought actors Daniel Radcliffe, Rupert Grint and Emma Watson - who is celebrating getting straight As in her A Levels in English literature, geography and art - will reprise their roles.
The release date for part one is tentatively set for November 2010.
Royal performance
Harry Potter and the Half-Blood Prince had been chosen for this year's Royal Film Performance, on 17 November.
Last year's performance was cancelled amid controversy about the chosen movie, Brick Lane.
It was the first time the annual gala - held in aid of the Cinema and Television Benevolent Fund (CTBF) - had been scrapped since 1958.
CTBF chief executive Peter Hore told BBC News he was "very disappointed" with the decision to shelve the Potter premiere.
But he was hopeful a Royal premiere would still go ahead this year.
He said: "The Royal Film Performance has been around for a long time and has a tradition of showcasing the best films. We are confident we will be able to do that again this year."
Mr Hore added that he expected to make an announcement shortly.
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