LONDON (Reuters) - J.K. Rowling's best-selling "Harry Potter" novels are available for the first time in ebook format on the new website dedicated to the boy wizard adventures.

The seven stories, which have sold an estimated 450 million copies worldwide and spawned a successful eight-film movie franchise, went on sale on Tuesday at the Pottermore site set up by Rowling.

The website's online store (shop.pottermore.com) is the exclusive retailer of Harry Potter ebooks and digital audio books, which have been launched in English only.

French, Italian, German and Spanish editions are expected in the coming weeks with further languages to follow.

The main Pottermore website is expected to be up and running in early April, several months later than initially anticipated owing to technical problems.

Designers hope to allow readers to explore elements of the Harry Potter world that have not appeared in the books and to interact with the stories and characters.

The free-to-use website, partnered by Sony, is one of several ventures launched by Rowling and her commercial partners to keep the magic of Harry Potter alive and the revenues rolling in.

Rowling, who recently announced she was turning to adult fiction, long resisted allowing the Harry Potter stories to be turned into digital format, but eventually decided that technological progress could not be stopped.

The ebook launch raises questions over copyright infringement for the jealously guarded Potter property, although organizers are using a combination of watermarking techniques to protect against illegal copying.

(Reporting by Mike Collett-White, editing by Paul Casciato)