Which Harry Potter novel is the longest in the UK first edition by page count?

Harry Potter and the Order of the Phoenix is the longest Harry Potter novel in the UK first edition, at 766 pages.

Bloomsbury published the fifth novel in the United Kingdom on 21 June 2003. The story follows Harry’s growing opposition to the Ministry of Magic, the formation of Dumbledore’s Army, and the return of the Order of the Phoenix as Voldemort’s threat becomes public.

Page counts vary between editions, languages, formats, and countries, so “longest” needs a stated basis. The commonly cited figure of 766 pages refers to the UK first edition. The American first edition has a different page count because of typesetting and design.

The novel is therefore longer than Goblet of Fire, although Goblet of Fire is also substantial. Deathly Hallows is the final novel but is not the longest by the standard UK comparison.