By Mark Savage
Entertainment reporter, BBC News
The final Harry Potter novel could hit shelves before its official release next week.
Author JK Rowling has said two characters will die in the book
Some shops are not expected to keep a written agreement which prevents them selling the book before 21 July.
"It's quite possible one will break the embargo," says Katherine Rushton from The Bookseller magazine. "They'd do it to be first, and for all the PR."
The book's publishers, Bloomsbury, said it had "no reason to believe anyone would want to ruin the excitement".
"If such a thing were to happen, we believe that the public would make their feelings known by not buying it from such a spoilsport retailer."
However, it added, "it is our intention to vigorously enforce the embargo if required".
Author JK Rowling has also stressed the importance of keeping to the official publication date on her website.
"I want the readers who have, in many instances, grown up with Harry, to embark on the last adventure they will share with him without knowing where they are going," she wrote.
Embargos on previous instalments of the series have generally been observed because Bloomsbury threatened to withhold future Harry Potter books from stores who flouted the rules.
This time there is no such incentive.
"Because it's the final Harry Potter, Bloomsbury doesn't have that same power," says Rushton. "It has played its last ace card."
Bookshops around the UK, many of which are opening at midnight to sell Harry Potter and the Deathly Hallows, have mixed opinions on whether the embargo will hold.
JK Rowling has amassed a £540m fortune from writing
"I'd be very surprised if anyone sold it early," said Tim Curtis of The Little Apple Bookshop in York. "I'm not sure consumers would want it."
"People like to be part of the midnight thing," agrees Suzanne Jones of Heffers in Cambridge.
"I've got an 11-year-old who has got it in her diary and she wants to go out and be part of it."
"To buy it in the cold light of day on a Friday is just not the same."
But Robert Webb, who has run Kingsthorpe Book Shop in Northampton for the last 34 years, says it is "a miracle" the embargo has held in the past.
"It wouldn't surprise me in the slightest if it did get broken this time.
"But I think the secret police are out there ready to jump on anyone who breaks the embargo with a terrific sledgehammer."
Hundreds of fans queued overnight to get hold of the last book
Harry Potter mania has led to the book becoming available in locations where you would never expect to find works of literature - such as garages and hardware stores.
It is thought that these shops - which have less to lose by souring relations with Bloomsbury - are the most likely to reveal the boy wizard's fate ahead of time.
"I don't think you'll get traditional bookshops doing that," says Rushton. "Inside the industry and outside the industry they'd be seen as spoilsports."
But the security operation surrounding the publication of the novel is so tight that anyone who does break the embargo is only likely to get a few hours head-start on their competitors.
They may also find themselves at the centre of legal action.
Everyone who stocks the book has had to sign an indemnity form, which makes them liable for damages and "other equitable relief" if they do not keep their side of the bargain.
As well as setting the embargo, the document imposes stringent conditions on retailers to ensure the plot of the book is not leaked.
The release date for the book was announced on JK Rowling's website
Retailers have to keep boxes containing the novel "in a secure area under lock and key", and they definitely must not be "visible to the public for any reason".
It is not even permitted to take a photograph of the books being delivered.
One shop-owner who will not be complying with the regulations is Mr Webb in Northampton - because he has decided not to order any copies of the last ever Harry Potter.
"There's a branch of Asda 100 yards away from our shop," he explains. "We're advising our loyal customers to take advantage of their generosity.
"They'll save three quid - which they can come and spend on something else here!"