The fifth Harry Potter instalment finally weighs in - at 768 pages

Children around the world will need to reinforce their school bags this summer. On 21 June, J K Rowling will finally publish Harry Potter and the Order of the Phoenix. A late delivery, the tome will be Rowling's biggest brainchild – by a margin of more than 200 pages.

Long awaited, and several times delayed, the fifth of her phenomenally popular series will appear three years after its predecessor. But rumours that the author, 37, had suffered writers' block were scotched yesterday when her publisher Bloomsbury revealed the length of the teenage wizard's latest adventures. Order of the Phoenix will weigh in at 768 pages and 255,000 words. Harry Potter and the Goblet of Fire, deemed freakishly long, managed a mere 191,000.

The children's literature specialist Nicholas Tucker said confident young readers would absorb the new blockbuster with glee. "Children have enormous powers of concentration," he said. "We're often talking about ourselves when we talk about their limited attention span." But he feared the book's bulk would prevent it being read aloud to younger fans. "It will actually eliminate the adult narrator," he said.

Goblet of Fire became the fastest-selling book in history on its debut in July 2000. Bloomsbury, which relies on Potter titles for half of its income, will be hoping to break more records. Rowling's four books have sold 192 million copies worldwide and can be read in 55 languages. Even without any new publication, they took the first three places in the 2002 children's bestseller charts. Meanwhile, revenues from the first film adaptation, Harry Potter and the Philosopher's Stone, are nudging a billion dollars.

After the announcement yesterday, Bloomsbury shares rose 27.5p, or 4 per cent, to 715p, while Scholastic, Rowling's US publisher, had advanced by 2.17 cents, or 6.3 per cent, by noon on Wall Street, trading as high as $36.82.

Rosamund de la Hey of Bloomsbury's children's division denied that the fattening girth of the books pointed to a trend in Rowling's work. The size of the sixth remains "completely anyone's guess, probably including hers", she said. So does its time of arrival: "She'll take as long as she wants."

Bloomsbury will disclose very little about the plot of the new book, though at some stage, Professor Albus Dumbledore – the headmaster of Hogwarts, Harry's school – will tell Harry fresh secrets about his background. According to the ancient logic of archetypal stories, these may well involve a closer relationship with the evil genius Lord Voldemort than Harry's admirers can yet imagine.