Enid Blyton, the author of the Famous Five and The Secret Seven, has fallen out of the top ten list of children's authors.
According to the latest figures, she has gone from position 8 to 11 in the chart and this is because youngsters cannot relate to her language, reports the Telegraph.
This has also been blamed on the fact that her books have not been made into any blockbuster films.
This year 464,000 copies of her works were sold marking a drop of 17.8 per cent for the period January to October 2010, said Neilsen BookScan.
Stephanie Meyer, the American who wrote the Twilight series, took the top spot followed by LJ Smith, who also writes about teenage vampires.
Rick Riordan, the author of Percy Jackson and the Olympians series, landed the third spot.
Jeff Kinney and Jacqueline Wilson were placed fourth and fifth.
A spokesman for Waterstones said: "The way the children express themselves in Enid Blyton can be a bit alienating for children today. The way the books are written and the words that are used a not necessarily relatable to nowadays. A lot of words have fallen out of common usage with children. You don't find that with someone like Roald Dahl who remains modern.
"Current writers have the advantage of movie adaptations and TV series and the celebrity that those bring with them. They also are able to generate fresh bouts of publicity whenever a new title is published, or when an author tours bookshops to publicise a new book to fans.
"Enid Blyton has done well to remain so popular after decades of being in print – there is no author with such a wide-ranging backlist who has lasted so long, but this last year has obviously seen her face competition from much younger writers. Blyton relies on her brand and the affection generations of readers have for her, and I am sure that in ten years time she will still be in the top 20 children's writers.
He added: "Roald Dahl benefits from frequent movie adaptations. His stories are also anarchic and undated, ensuring they seem fresh to every new generation discovering them. His language feels very current as well as the stories whereas it could be argued that Enid Blyton seems a bit dated."