Which Harry Potter novel is the longest in the United Kingdom edition?

Harry Potter and the Order of the Phoenix is the longest Harry Potter novel in the United Kingdom edition.

The fifth book was published in 2003 and contains 766 pages in its standard UK hardback edition. It follows Harry’s fifth year at Hogwarts, the Ministry of Magic’s denial that Lord Voldemort has returned, and the formation of Dumbledore’s Army.

The novel is longer than the other six main books in their standard UK editions, although page counts can vary between hardback, paperback, large-print, and translated versions. That is why the claim is best stated with the edition context included.

Readers sometimes select Goblet of Fire because it is also a large volume and marks a major turning point in the series. Deathly Hallows is the final novel, but it is shorter in the standard British edition than Order of the Phoenix.