FOR years, the story of a child wizard languished in Angie Sage’s desk drawer, but the commercial success of Harry Potter made her realise he too could have mass appeal.

Now Sage, a book illustrator living in Milverton, Somerset, is Britain’s latest Hollywood success story, with a deal for the screen-plays of all seven books in her series about the wizard, Septimus Heap. He writes spells on slices of toast and can turn anything to chocolate.

Warner Brothers finances the Harry Potter films and considered hundreds of fantasy books before signing the deal with Sage, in preparation for the end of the Potter movies in the summer of 2010.

JK Rowling will launch the seventh and final book in her series at the Natural History Museum at midnight on Friday. She will sign copies of Harry Potter and the Deathly Hallows for 1,700 competition winners.

So far, the first six books in the series have sold more than 325m copies worldwide, generating a fortune for Rowling calculated at £545m in The Sunday Times Rich List. The sixth, Harry Potter and the Half-Blood Prince, sold 2,009,574 in the UK on the first day of its release, a record.

Sage, 55, admits she is surprised by her fame in America where her novels of magic and mayhem have soared to the top of the book charts, driven by the internet and word of mouth. She is currently writing the fourth book and the film deal could potentially be worth millions.

The author, who until recently lived in Mylor on the Cornish coast, the atmosphere of which pervades much of the books, will be an executive producer on the Warner Brothers films so she can preserve the British tone of her magical saga.

“We have been talking for more than a year, but I was not able to say anything until now,” Sage said this weekend. “It still does not feel real. I am in shock really, but it seems to be moving very fast.”

The first book, Magyk, tells how the infant Septimus was kidnapped from his family and replaced by a princess. It will be adapted for the screen by Karen Rosenfelt, who produced last year’s hit The Devil Wears Prada.

Sage started writing the Septimus Heap books in the late 1990s, before Potter became a global success, but kept hiding her work away in the bottom drawer of her desk. “I just kept pulling it out, working on it a bit, and then putting it away again,” she said.

“I really need to thank JK Rowling for showing us all that children will read magical stories and giving me the confidence to send my manuscript off to the publisher. But, as far as I can tell, although it’s also set in a kind of magical England across seven books, that is as similar as the books get.”

She saw the first Harry Potter film and then stopped taking notice of the Rowling phenomenon to avoid being influenced. She has also avoided reading His Dark Materials, the trilogy by Philip Pullman, to whom she has also been compared. Films of the trilogy are now in progress with Nicole Kidman as the villain Marisa Coulter.

Although Septimus Heap does have a skeletal bogeyman in the shape of Dom Daniel, a necromancer, the books are characterised by a gentle humour expressed through spells such as the Reverse, which turns objects into their opposite, or the Shield Bug, which creates a tiny insect armed with a sword to defend the spell-caster.

Sage hopes to finish the series by 2012, when she turns 60. She has the end already in mind, but still occasionally gets stuck. Then she writes all the names of the characters on slips of paper, shakes them up in a matchbox and takes inspiration from the order in which they fall out.

Other Hollywood deals for British writers include the purchase by Disney of children’s books from Clive Barker, a horror novelist from Liverpool. Neil Gaiman, a fantasy author, has written the script for a version of Beowulf to star Angelina Jolie. Eoin Colfer’s Artemis Fowl books, described as “Die Hard with fairies”, is under consideration at several studios.

“Nobody does goblins like the Brits,” said one Warner executive. “I am not sure it’s healthy. But the world sure love those wizards and ghoulies. It’s your biggest export since the Beatles.”

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