Prequel Harry Potter

JK Rowling has written an 800-word prequel to the Harry Potter series which is likely to raise hundreds of thousands of pounds for charity at auction.

The handwritten short story, featuring the fictional wizard's father James and his friend Sirius Black in their younger years, fills both sides of a piece of A5 card and is signed by the author. A 93-word teaser card carrying plot clues sold in 2002 for £26,680, and The Tales of Beedle the Bard, a book of fairytales which plays a central role in Harry Potter and The Deathly Hallows, made £1.95m last year.

Rowling, who has earned £560m from the seven Potter books, is determined the latest instalment will not be developed further, writing at the end of the story: "From the prequel I am not working on - but that was fun!" Last year she said that if she did write another Potter tale it would not be for a decade.

The new work is among 13 storycards by authors including Margaret Atwood, Nick Hornby and Doris Lessing being auctioned to raise money for English Pen, which promotes understanding through literature, and Dyslexia Action at Waterstone's London Piccadilly store next month.

Copies of the What's Your Story? cards will be displayed in branches and online after the sale and will be published as a collection to be sold by the chain in August, with profits going to the charities.

Gerry Johnson, managing director of Waterstone's, said: "We never dreamed that JK Rowling would donate something so precious. I can't begin to guess how much it will raise at auction." The plot is a closely guarded secret, but Philip Errington, children's books specialist at Sotheby's, which is conducting the auction on June 10, revealed that Harry does not feature, although a policeman does.