Tesco book buyer Caroline Ridding said: "The sixth Harry Potter has become the most eagerly anticipated book in literary history and has already broken our pre-release sales.
"We are expecting it to continue to smash all our sales records as it becomes the biggest and fastestselling book we have ever stocked."
Both Liverpool and Southport branches of Waterstone's booksellers are also opening at the stroke of midnight, and early on Saturday, to sell copies of Harry Potter's latest adventure.
A spokeswoman from the National Literacy Trust, which aims to promote reading and literacy in the UK, said: "The Harry Potter books are unique because they appeal to such a wide group of readers - including many people who would not be typically interested in the adventures of a public schoolboy."
Despite the tight security surrounding the release of the book, a number of copies were inadvertently sold ahead of the strict release date in New York and Vancouver.
Meanwhile, two senior Government figures added their praise to the Harry Potter books.
Chancellor Gordon Brown - a personal friend of the author - said: "It's one of our greatest exports and one of our greatest success stories."
His comments and those of another fan, David Blunkett, Work and Pensions Secretary, will be shown tonight in an ITV1 show, Harry Potter at the Castle: Magic at Midnight.
