A mini-prequel penned by JK Rowling to her blockbuster "Harry Potter" series of books will go up for bids in London next month, the auction house behind the sale said Thursday.

The auction of the 800-word short story comes less than a year after the seventh and final tome in the series about the bespectacled boy wizard, "Harry Potter and the Deathly Hallows" went on sale.

"There is just no telling how high the bidding might go," said Sotheby's auction house specialist Philip Errington.

The proceeds of will go to two charities: English PEN, which promotes the freedom to write around the world, and Dyslexia Action.

Organizers of the auction had asked 13 authors to submit a story for the sale.

Among them were Doris Lessing, the 2007 winner of the Nobel Prize for Literature; Booker Prize winner Margaret Atwood; and Sebastian Faulks whose latest offering, an officially authorized James Bond story, went on sale Wednesday.

Rowling's "Harry Potter" series has sold nearly 350 million copies around the world in 65 languages and been turned into a successful film franchise.