Which J.K. Rowling novel was the longest of the original Harry Potter series by UK page count?

Harry Potter and the Order of the Phoenix was the longest original Harry Potter novel by UK page count.

The UK hardback edition runs to 766 pages, making it the largest of the seven novels in the British print editions commonly cited for the series. It was published in 2003 and follows Harry’s fifth year at Hogwarts, during which the Ministry of Magic refuses to accept that Lord Voldemort has returned.

The novel introduces Dolores Umbridge as the Ministry’s appointed Defence Against the Dark Arts teacher. Because she limits practical instruction, Harry and his classmates form Dumbledore’s Army to practise defensive magic together.

Page counts vary between formats, countries, fonts, and editions, so “longest” is most precise when tied to the standard UK hardback comparison. The Goblet of Fire is also a long volume, but its commonly cited UK hardback count is lower.