The creator of the best-selling Alex Rider spy novels has accused fellow children’s author David Walliams of dumbing-down fiction and failing to ‘challenge’ young readers. Anthony Horowitz said books by the comedian – who was the UK’s top-selling children’s author last year – are ‘witty and entertaining’ but nowhere near ambitious enough. The 59-year-old novelist and screenwriter singled out Walliams’ Gangsta Granny for criticism, as well as the Diary Of A Wimpy Kid books by author Jeff Kinney. Scroll down for video . Author Anthony Horowitz (pictured right) said books by comedian David Walliams (left) – who was the UK’s top-selling children’s author last year – are ‘witty and entertaining’ but nowhere near ambitious enough . He insisted that such writers should challenge their young readers and not be ‘afraid of powerful stories or serious ideas’. He suggested they should follow the example of authors such as John Green and ‘write up for children, not down to them’. Green’s book, The Fault In Our Stars, tells the story of two teenagers who fall in love while they are both dying of cancer, and was the top-selling title of 2014, with more than 870,000 copies sold. Horowitz, who is also the principal writer on the ITV period detective drama Foyle’s War, singled out Gangsta Granny, with its breaking-wind jokes and a character who perpetuates the myth that children ‘automatically dislike reading’. Writing in the Times Educational Supplement, he said he was troubled that children’s books ‘seem to have come full circle’. Walliams made over £7million in 2014 from book sales including The Boy in the Dress and Demon Dentist . He added: ‘To some extent, narrative fiction was reinvented by JK Rowling – it’s hard to believe that children weren’t challenged by books that stretched to 760 pages – and a phalanx of writers crested on her success: Michael Morpurgo, Philip Pullman, Jacqueline Wilson, Malorie Blackman, Eoin Colfer, Darren Shan... and me. 'Today it is Jeff Kinney and David Walliams who top the bestseller lists, with books that are witty and entertaining but nowhere near as ambitious.’ He added: ‘Speaking personally, I was a little sad when, in Walliams’ Gangsta Granny, we were told that among the old woman’s vices, which included dribbling and farting: “Her house was stuffed full of books and she was always trying to get Ben to read them, even though he loathed reading”.’ Horowitz said ‘the idea that children automatically dislike reading or that books belong to a more fusty, ancient generation is patently absurd’. His Alex Rider books, the first of which was published in 2000, have been ‘brightly re-jacketed’, and are ‘as popular as ever’. They have sold 19million copies in 30 different languages, he pointed out. Horowitz said: ‘Quite recently, I addressed an audience of largely unpublished writers in New York and I stand by the advice that I gave them. ‘Write up for children, not down to them. We don’t need to be afraid of powerful stories or serious ideas. The audience is there.’ Walliams last year made more than £7million from book sales. The 43-year-old Britain’s Got Talent judge had four books in last year’s top 50 bestsellers list. His latest book, Awful Auntie, sold more than 500,000 copies last year. The success of his books, which also include Ratburger and Billionaire Boy, has also been boosted by television adaptations, such as The Boy in the Dress, which starred Walliams, Meera Syal and Jennifer Saunders, and was shown on the BBC at Christmas.
Author Anthony Horowitz has accused David Walliams of dumbing down .
Claims Walliams fails to challenge young readers with unambitious books .
Horowitz argues authors should not be afraid of 'powerful stories or ideas'