According to online retailer Amazon, JK Rowling's seventh Potter book has become its most pre-ordered product.
Demand has surpassed a previous record of 1.5 million copies ordered ahead of the release of The Half-Blood Prince.
The latest film in the series, The Order of the Phoenix, has its European premiere in London on Tuesday.
The film opens across the globe on 11 July.
It is believed that author Rowling will join the film's stars, including Daniel Radcliffe, Imelda Staunton and Robbie Coltrane, on the red carpet in Leicester Square.
Guests including Doctor Who star David Tennant, actor Pierce Brosnan and presenter Graham Norton are also expected to attend.
Speculation
The world premiere of the film took place in Tokyo, where Harry Potter has a huge fanbase.
Another highly-anticipated sequel, Spider-Man 3, opened in the Japanese capital in April and went on to break box office records.
Rowling has said two characters die in the final book, prompting worldwide speculation.
The Potter books have sold 325 million copies worldwide, have been translated into 64 languages and spawned five films.