The penultimate chronicle of the boy wizard’s life at Hogwarts School of Witchcraft and Wizardry will be released at midnight on Saturday to cheers from children and howls from accountants.

Harry Potter and the Half-Blood Prince has started a huge price war as shops sacrifice profit for market share. The two biggest price-cutters yesterday were Asda and Tesco’s online service, both of which offered the book for £8.96, a 47 per cent reduction on the recommended retail price of £16.99.

The price is likely to be reduced again by the weekend as retailers compete to have the best advertised price. Asda has promised to refund the difference to anyone who finds a cheaper copy elsewhere.

For the previous instalment, Harry Potter and the Order of the Phoenix, Tesco.com reduced its price to £7.64 — a discount of 55 per cent.

More than 1.7 million readers will buy the book in the first 24 hours and an estimated 3.2 million will buy it within the first week, a feat unmatched by any author in Britain.

John Grisham and Terry Pratchett, the most popular writers of adult fiction, will sell no more than 20,000 books in the first week of sale.

Bookshops and supermarkets know that if they can get just 0.6 per cent of first week sales of Harry Potter, it is equivalent to having a monopoly on John Grisham. Supermarkets have the added bonus that readers are also likely to spend money on other products and will associate the shop with their favourite book.

Supermarkets believe this so strongly that they are prepared to sacrifice millions of pounds in profits. If all retailers sold the book at full price, they would make an estimated £68 million. If the average price is £9.99, they will make £40 million, a loss of £28 million that goes straight to the consumer.

Most retailers refused to state their final price in the hope that their rivals will hold firm, preventing an expensive spiral of discounts.

Bloomsbury, J. K. Rowling’s publisher, does not declare the price at which it sells the books to retailers, but it is likely to be 55 per cent lower than the cover price. Supermarkets that promise to have the lowest prices cannot sink much below £7.64 before they make a loss.

Rosamund de la Hey, former head of children’s books at Bloomsbury, has said that she was powerless to prevent discounting. “Bloomsbury would love to be in a position to restrain discounting but there’s little we can do. It is all down to the individual retailers.”

Jo Marino, head of communications at Waterstone’s, said that the shop was determined not to cut its price below £11.99.

The chain is instead offering a free copy of Lionboy, by Zizou Corder, with every purchase. “It is not just about a price war,” she said.

“I think that is where other shops have forgotten what Harry Potter is all about.”

Jon Howells of Ottakars bookshops predicted that there would be more than 2.5 million copies in the first print run, easily the largest in British history.

The first print run of Harry Potter and the Philosopher’s Stone, the original adventure, was 2,000. “There is no comparison with other books,” he said. “We don’t have records for first-day sales of other books because people don’t queue up for them in the same way. The next time we will see something like this is for Dan Brown’s next book after The Da Vinci Code. That will probably sell between 50,000 and 100,000 copies in the first day.”

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