site received more than 70,000 visitors in the first 20 hours of the story going live online last week and has continued to attract thousands more every day.
Rowling's Harry Potter story is set three years before Harry is born and features Sirius Black and James Potter. In the 800-word story, the two men are being chased by the police after breaking the speed limit on their motorbikes.
Rowling's exclusive story is one of a number of stories auctioned off at Sotheby's, raising over £25,000 for charity as part of the What's Your Story? competition.
Waterstone's promotion is searching for budding writers to be publishing alongside Rowling, Nick Hornby and other authors. To be in with a chance of winning competitors simply enter their story into the online story card template. The competition has so far attracted nearly 4,000 story cards.
This latest promotion follows the success of Waterstone's Save Harry Potter online petition, which garnered achieved more than 500,000 signatures last year.