Published by Simon & Schuster, Return to Ribblestrop is the sequel to Mulligan's first ever book, 'Ribblestrop', and tells the adventures of the pupils and teachers at Ribblestrop Towers.
Established in 1967, the Guardian children's fiction prize is unique in that it is the only children's prize to be judged by writers, and because authors are only able to win the prize once. Andy joins a distinguished group of past winners including Anne Fine, Philip Pullman, Jacqueline Wilson, Meg Rosoff and Patrick Ness.
This year's winner was chosen by children's authors Julia Golding and Marcus Sedgwick, along with the prize's 2010 winner, Michelle Paver. The judging panel was chaired by the Guardian Children's Books editor, Julia Eccleshare. Young readers taking part in the Guardian's 2011 Young Critics competition - as well as members of the Guardian's children's book site, which launched earlier this year - also had their say as they judged the longlisted books.
Julia Eccleshare said: "The judges considered energetically and argued passionately in their search for this year's winner of the Guardian Children's Fiction Prize, but Return to Ribblestrop - a school story with a cornucopia of differences - had just the right combination of warmth, originality and, above all, hilarity to give it the edge.
"With its huge cast of unusual and big-hearted characters, combined with the strong streak of total anarchy that runs through it, Return to Ribblestrop epitomises the unlimited scope that the best books possess to entertain a new generation of readers."
Andy Mulligan said: "It is very hard to take yourself seriously as a writer when you've always written for fun. You don't really expect what you write to connect with people the way other writers have connected with you.
"I wrote Return to Ribblestrop firmly convinced that nobody would get it, so the thought that people have, and that people regard it as good, is a little bit overwhelming. I never expected the Guardian to award such a stonker of a prize to a book that is dangerous, violent, irreverent, politically incorrect, joyously sentimental, anti-adult, pro-child and sometimes bizarre - but I'm very glad they have."