The transformation has been overseen by JK Rowling, the author of the series, and will be the centrepiece of one of the most expensive and extravagant launch parties in the history of publishing.

On Friday evening the young guests and their parents or guardians will be collected from a reception at the City Chambers in Edinburgh by 12 carriages drawn by black horses adorned with black ostrich plumes and driven by coachmen dressed in capes.

Rowling plans to make her appearance at 12.01am from behind a secret panel to read the first chapter of the new book, a copy of which will be given to each child.

The following evening, the children will be “sorted” into one of the four Hogwarts houses — Gryffindor, Hufflepuff, Ravenclaw and Slytherin. The guests will then be assigned to a house table, lined with benches, before wizard-themed food is served on gold platters.

The banquet will take place in the castle’s Jacobite Room, which has been hired at a cost of more than £500 an hour and will be decorated with school banners and illuminated with torches. The food for the Hogwarts-style banquet has been devised by Digby Trout, Edinburgh Castle’s caterers, whose basic menu starts at £39 a head.

The caterers have also developed their own recipe for butter beer. The non-alcoholic mulled cider is first drunk by Harry in the third book in the series and is described by Rowling as tasting “a little bit like less sickly butterscotch”.

“JK Rowling has been very much involved — she always wants events to be focused on the children. She has been consulted at every stage.” said Lucy Holden, spokeswoman for Bloomsbury. "Everything we serve will be along a Hogwarts theme- we've even had to create a few new recipes," she said.