Set for release Thursday, the book is expected to be one of the biggest sellers of the year. Rowling previously penned the seven-book Harry Potter series, which was adapted as a blockbuster film franchise.
The current record-holder for most hardcover sales of an adult novel in a first week in the United Kingdom is "The Lost Symbol" by Dan Brown, which sold 550,000 copies its first week in 2009, The Telegraph said.
"This is definitely the biggest launch of the year. Having said that, it is not in the league of Harry Potter, which is obviously a phenomenon that built up over a decade," a Waterstone's bookseller spokesman told the newspaper.