LONDON, England (CNN) -- Harry Potter author J.K. Rowling is writing a prequel to her best-selling series  to be auctioned for charity -- but at just 800 words, it may lack some of the magic fans of the boy wizard might be hoping for.



Autographed copies of J.K. Rowling's work have fetched millions of dollars at auction.



Rowling's hand-written prequel, signed by the author, will be auctioned alongside works by other famous writers to raise cash for a dyslexia charity.



Despite the brevity of the piece, experts believe it could fetch big money, since other autographed works by the blockbuster author have sold for millions of dollars.



"We never dreamed that J.K. Rowling would donate something so precious, and we're incredibly grateful," said Gerry Johnson, managing director of Waterstone's, the UK book chain organizing the auction.



"I can't begin to guess how much it will raise at auction."



A previous 93-word storycard from Rowling, which referred to the book "Harry Potter and the Order of the Phoenix," sold in 2002 for $53,000. Rowling later produced seven hand-written copies of a new work, "The Tales of Beedle the Bard," one of which sold for $3.9 million in 2007.



"Given the enormous interest we have seen in recent times for autograph work by J.K. Rowling, the prospects for her storycard are good to say the least," said Philip Errington, a specialist at Sotheby's auction house, which is helping to organize the sale.



The card on which the story is written -- measuring 14.6 by 20.9 centimeters (5.75 by 8.25 inches) -- is signed "JK Rowling 2008." The author signs off with the message: "From the prequel I am not working on -- but that was fun!"



Alongside Rowling, 13 other authors invited to contribute to the June 10 auction include Margaret Atwood, Sebastian Faulks, Nick Hornby, and Tom Stoppard. A book featuring all 13 cards will go on sale in August.



All profits from the book will benefit Dyslexia Action and English PEN, a writers' association, Waterstone's said.
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