Customer Mandy Muldoon said she simply spotted a stack of the highly anticipated books on a shelf and bought one. She has since promised to return it. Shop manager Christine Ekblom said she believed it was the only copy sold. "They were pulled off an hour after they were put on the shelves. It was a mistake," she said. A gag order was issued by a Canadian court this week after 14 copies of Harry Potter and the Half-Blood Prince were inadvertently sold at a supermarket near Vancouver.

The book is due for release this Saturday and will be unveiled at the stroke of midnight at special parties held at bookshops worldwide. Ms Muldoon’s husband, Mike, said the family would return the book to the publisher, although his stepson had admitted reading two pages. "We’re going to do the right thing," he said. "We don’t want to ruin it for other kids and take away from the experience of everyone reading it together." Kyle Good, a Scholastic publishing spokeswoman, said: "This is the first early sale we have heard of in the US and the family is returning it." On Friday, Scholastic will hold a ceremony to unveil the first signed copy of Harry Potter and the Half-Blood Prince – transported to the US on the QE2 in a gigantic locked trunk. The book will be presented to a public library.