Which Harry Potter book is the longest in the British editions?

Harry Potter and the Order of the Phoenix is the longest Harry Potter book in the British editions.

Bloomsbury published the fifth novel on 21 June 2003. The British hardback edition has 766 pages, making it longer than the other six novels in their original British book formats. The story follows Harry’s fifth year at Hogwarts and the Ministry of Magic’s refusal to accept that Lord Voldemort has returned.

The novel introduces the student group known as Dumbledore’s Army and features Dolores Umbridge as the Ministry-appointed Hogwarts professor. Harry also experiences increasingly troubling visions linked to Voldemort.

Page counts vary between editions, languages and formats, so comparisons must specify the edition. In the original British editions, however, Order of the Phoenix is the clear longest volume by page count.