Which Harry Potter book is the longest by page count in its original UK edition?

Harry Potter and the Order of the Phoenix is the longest Harry Potter book by page count in its original UK edition. The Bloomsbury hardback edition contains 766 pages.

The fifth novel was published on 21 June 2003 and follows Harry during his fifth year at Hogwarts. Its larger size reflects several major plotlines, including the Ministry of Magic’s campaign against Harry’s account of Voldemort’s return, the formation of Dumbledore’s Army, and the battle at the Department of Mysteries.

Page counts vary between countries, formats, type sizes, and editions, so “longest” needs a stated basis. The commonly quoted comparison uses the original UK hardback editions. By that measure, Order of the Phoenix exceeds Goblet of Fire and Deathly Hallows, even though other editions may have different pagination.