Theresa May's plans to create a new generation of grammar schools are "quite deeply stupid" and will create vast numbers of children who "fail, and fail young," according to War Horse author Michael Morpurgo.
Education Secretary Justine Greening has insisted expanding selective schools could help improve pupil attainment, but Mr Morpurgo, who failed his 11-plus exam when he applied to go to grammar school, said the proposals are "perpetuating a myth".
He told the Press Association: "I don't think it's a conspiracy but I think it's quite deeply stupid to think if you educate people in such a way that they are divided when they are young, you aren't going to create two societies.
"Some people are on the fast-track to fulfilment and prosperity and there are other people who get left behind."
The award-winning children's author and former Children's Laureate was sent to an independent school thanks to funding from "kindly aunties", but said his experience of failure at an early age was devastating.
He said: "Failure is the worst thing you can do to a child, it crushes their confidence. I condemned myself because of this failure, you were named and shamed, you knew you had disappointed everyone.
"I was knocked back and went off in the other direction. I did sports and music, not academics, and when I came to exams I had no confidence, and I trace it back to failing the 11-plus so openly.
"I know from being on that side of it, it is not the way to go.
"There are grammar schools that are wonderful but there are also great comprehensives and academies."
Mr Morpurgo said the plans only add to divisions that already exist in society, adding: "We know this from Brexit, we are so divided now, we are so not together.
"Our class system has always favoured areas of great prosperity and you can't blame parents for wanting that for their children, but you have to pay attention to the half that don't succeed.
"You will have vast numbers of children who fail, and fail young.
"Other countries do it much more wisely, separating children according to talent when they are older. It is still tough, but it is not the same as at 11.
"In her speech outside Downing Street, Theresa May said she wanted this country to have opportunities for everyone. You don't create opportunities by creating failure."
Mr Morpurgo is famous for his books including War Horse, Private Peaceful, Friend Or Foe and Why The Whales Came, which have all been adapted for the big screen.
The stage adaptation of War Horse, which features life-sized horse puppets, premiered at the National Theatre in 2007 and is still touring the world.
The author will read extracts from the book accompanied by the Royal Philharmonic Concert Orchestra at a special production at the Royal Albert Hall on October 27, while British designer Rae Smith will draw scenes from the book live on stage.
He said: "I hope people will come because they might have read the book or been to the National Theatre or seen the movie. The music shapes the whole thing together and that is what we are going to be seeing."