Harry Potter author JK Rowling says the seventh book in the boy wizard series, due for release in July, may not be the last in the series after all.

Despite insisting the story of Harry Potter would end after the release of the seventh book, the author has unveiled she has another instalment in mind.

"I might do an eighth book for charity, a kind of encyclopedia of the world so I could use all the extra material that's not in the books," she has been reported as saying.

The seventh book in the hugely popular series, Harry Potter And The Deathly Hallows, is released worldwide on July 21.