Author JK Rowling's prequel to Harry Potter fetched £25,000 at a charity auction today

JK Rowling's prequel to Harry Potter sold at auction for £25,000 tonight amid frenzied bidding by fans of the boy wizard.

The story, which had been shrouded in secrecy, was penned last month and is just 800 words long.

Her Potter prequel comes almost a year after the last book in the series, Harry Potter And The Deathly Hallows, hit the shelves.

Organisers of the charity auction asked 13 authors, including JK Rowling, Margaret Atwood, Sebastian Faulks, Nick Hornby and Doris Lessing, to create a story for the sale.

The celebrated writers were told they could submit anything from a doodle and collage to a fresh story.

But JK Rowling, who has earned a £560 million fortune from the seven Potter stories which sold more than 375 million books worldwide, is determined the latest instalment will not be developed further.

She says at the end of the story, written on both sides of an A5 storycard: "From the prequel I am not working on - but that was fun!"

£25,000 storycard: The 800 word storycard was hand-written and signed by JK Rowling for a unique and rare memento of the Harry Potter Series

The story is set three years before Harry is born and features the characters Sirius Black and James Potter, Harry's father.

They get in a scrape with irate policemen before making their escape via broomsticks, drumsticks and a little bit of magic.

The author said tonight: "I had great fun writing the card and I'm absolutely delighted that it has raised £25,000 for two such worthy causes."

JK Rowling, whose books have also been turned into a series of record-breaking films, has previously stated that if she did write another Potter tale, it would not be for another 10 years.

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Other high bids from tonight's auction included Tom Stoppard's card, which sold for £4,000, and Doris Lessing's which raised £3,000.

Waterstone's managing director Gerry Johnson said: "We never imagined when Waterstone's started What's Your Story? that we would end up with authors who have won the Booker, the Nobel Prize and so many other accolades, great storytellers one and all, including the writer of the bestselling books in history."

Money raised from the sale will go to English PEN, which promotes understanding through literature, and Dyslexia Action.

In December last year, a new JK Rowling work, The Tales Of Beedle The Bard, fetched £1.95 million at auction.

The auction was held at Waterstone's in Piccadilly, London.