A spokesman for St John's said: "The school held an assembly on the last day of term to say goodbye to the children and staff who were leaving.

"At this assembly a very small passage from the new Harry Potter book was read aloud.

"The passage was carefully chosen to reflect the theme of saying goodbye.

"The school felt that this reading would not spoil the children's enjoyment of the book and its plot.

"Many of the children and staff at the school are fans of the Harry Potter series, and we used the text as a way of illustrating how uncertainty can sometimes accompany new beginnings."

Spokesman Margaret Morrissey said: "It beggars belief as to how someone could even consider doing something like this. It spoilt the book for a number of children.

"The good thing about Harry Potter is that it has encouraged a lot of children to pick up books and read.

"Whatever the motive there wasn't sufficient thinking behind this.

"It is something that the children at the school would have spent the majority of the holiday reading rather than saying they were bored.

"It is very hard to understand why anyone would do that, most of all a teacher in a primary school."