Which Harry Potter book was the first to sell more than one million copies in its first 24 hours in the UK?

Harry Potter and the Order of the Phoenix was the first Harry Potter book to sell more than one million copies in its first 24 hours in the United Kingdom.

The fifth novel was published in 2003, three years after Harry Potter and the Goblet of Fire. Its release became a major publishing event, with midnight sales, extensive bookstore promotions, and intense worldwide attention surrounding the next stage of Harry’s conflict with Voldemort.

The book is the longest in the original series by word count in its standard English editions. It introduces the oppressive Hogwarts regime of Dolores Umbridge and develops the student resistance group known as Dumbledore’s Army.

Later books broke or exceeded further sales records, especially Harry Potter and the Deathly Hallows. Those later achievements can cause confusion, but the one-million-copy first-day UK milestone belongs specifically to Order of the Phoenix.