London: A Harry Potter spin-off by JK Rowling was predicted to become the biggest selling book of the year in Britain when it went on sale on Thursday.

Pre-orders had already shot The Tales of Beedle the Bard - a charity venture - to the top of this week's chart, said booksellers. Philip Stone, charts editor at The Bookseller magazine, said: "I think it's an obvious choice for the Christmas No 1. It will probably sell more than 650,000 copies by the end of the year."

That would knock the current 2008 No 1, Linwood Barclay's novel No Time for Goodbye, off the spot. Barclay's book, recommended by the powerful Richard and Judy Book Club, has sold 615,000 copies to date, according to Nielsen BookScan.

In July 2007 Harry Potter and the Deathly Hallows, the seventh and final in the series, became the fastest selling book ever. It sold 2,652,656 copies on its opening day.

Stone said he did not think The Tales of Beedle the Bard, a collection of short stories, would surpass sales of Deathly Hallows.

But publishers are expecting a worldwide hit: they have printed eight million copies. All net proceeds - £1.61 for the standard £6.99 British edition - will go towards the charity that Rowling helped set up, called The Children's High Level Group (CHLG). It helps vulnerable children across Europe. If all copies are sold then the venture could make it almost £15 million.

Hand-written

Rowling originally hand wrote just seven copies of the book, giving six to friends. The final copy, bound in leather and embossed with silver and moonstone, was bought by the online bookseller Amazon for £1.95 million last December. It was only expected to fetch £50,000. The money went to The Children's Voice, the predecessor of CHLG. Months of frantic internet rumours followed about whether Beedle would be put into print.

In August, the publisher Bloomsbury confirmed it would be printing a British version while Scholastic would produce a US version. Amazon also announced it would be printing a 100,000-copy collector's edition, based on the leather-bound original, that would sell for £50.

A spokesman said most of those had been pre-ordered with only "a few copies left".