Education secretary Justine Greening has insisted that expanding selective schools could help improve pupil attainment, but Mr Morpurgo, who failed his 11-plus exam, said the proposals were "perpetuating a myth".
"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,” the author of War Horse said.
"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.
'Crushes their confidence'
"Failure is the worst thing you can do to a child,” he told the Press Association. “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."
'Fail, and fail young'
Mr Morpurgo said the plans only add to divisions that already exist in society. "We know this from Brexit,” he added. “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.”
He added that other countries introduce selection much more wisely, separating children according to talent when they are older. “It is still tough,” he said, “but it is not the same as at 11.
"Theresa May said she wanted this country to have opportunities for everyone. You don't create opportunities by creating failure."