There is no magic formula when it comes to selling books

Ottakar's has been selling copies of Harry Potter and the Half-Blood Prince for �11.99. Supermarkets have sold the same book for as little as �4.99.

The firm said same-store sales dropped 6.7% in the 4 weeks to 16 July, an announcement that hammered its shares.

The figure includes first day sales of the Half-Blood Prince, the sixth book in the series about a young wizard.

Record breaker

Released over the weekend amid the sort of hype that is usually reserved for a Hollywood blockbuster, the book sold almost nine million copies in its first 24 hours.

Ottaker's said that it sold about 70,000 books, less than the 100,000 it had expected to offload.

By comparison, supermarket chain Tesco turned over close to 400,000 copies.

Such low prices have been both a cause and effect of greater consumer popularity The Bookseller

Ottaker's founder and managing director James Heneage said that the release of a Harry Potter book is a unique event that casts "a long shadow".

Sales were also hit because publishers were reluctant to release books ahead of or after the Potter tome, unlike last year when the biography of Bill Clinton and the latest from Louis de Bernieres got tills ringing, Mr Heneage said.

Brand 'stolen'

The expectations surrounding the release of any new Harry Potter book are now so high that firms which encounter problems are immediately hit by investors.

Shares in Ottakar's dropped 9.3% to 260 pence in London after announcing the sales problems.

Analysts and industry observers have been examining the effect that supermarkets are having on the book market.

"Such low prices have been both a cause and effect of greater consumer popularity, but many independent booksellers now view Potter with cynicism," industry magazine The Bookseller said on its website.

"The discounting means that many make next to nothing from the boy wizard, while they feel that a brand they helped create has been stolen by the supermarkets."

Ottaker's Mr Heneage is less negative, and said that supermarkets have been helping rather than hindering sales at book stores.