Rowling's fantasy series, most recently "Harry Potter and Half-Blood Prince," has sold more than 200 million copies worldwide in print editions and more than 5 million as audiobooks, narrated by Grammy winner Jim Dale.

But up to now the author had only permitted paper and traditional audio releases, making her work a favourite for online pirates, although illegal sales are believed to be relatively tiny.

Helped by the iPod boom, digital audiobooks are already one of publishing's hottest sectors, with sales nearly quadrupling between 2001 and 2003, to more than US$18 million, according to the Audio Publishers Association.

"It's very exciting that an audiobook both critically acclaimed and commercially successful is finally available to the very broad audience of people who enjoy downloading," says association president Mary Beth Roche.

Also on Wednesday, Rowling said on her website that she was concerned by a wave of Potter merchandise with fake autographs for sale on eBay. But eBay spokesman Hani Durzi said Rowling is part of a copyright protection programme offered by the online auction giant that allows members to report problems.