Demand for the new book is expected to be just as high as last year when 500,000 pre-ordered copies of the sixth book were delivered in Britain.
A Royal Mail spokesman commented: "This kind of operation takes a lot of planning."
A new Harry Potter book release is one of the busiest times of the year for postal workers, second only to Christmas.
Author JK Rowling said recently that she had nearly finished the last book but would not reveal any plot details, although she has admitted two characters will die.
The first four novels in the series, about a young wizard and his friends, have been turned into films and sales of all Harry Potter books now total more than 300 million worldwide.
