Sorting pupils as young as four by ability limits their expectations and is one of the most distressing education policies of the current government, according to Jo Boaler, associate professor of mathematics education at Stanford University, California.
Young children put in lower sets, who come disproportionately from poorer backgrounds, are consigned to a psychological prison which breaks their ambition and from which they find it almost impossible to escape.
Mixed-ability teaching, much maligned by politicians, is difficult but worth the extra effort because it is fairer and produces better overall results, she said.
Setting of young pupils has been rejected by other countries and research by the Organisation for Economic Cooperation and Development shows schools that divide students latest and least have the best results.
Dr Boaler accused Labour of going further than the Conservatives in its attack on mixed-ability teaching.
She said: "The fact that our children's future is decided for them by the time they are four years old derides the work of schools and contravenes basic knowledge about child development and learning.
"If the Labour Party really cares about promoting social justice then (it) must learn about equitable and effective grouping policies that promote high achievement for all rather than reproduce social inequalities."
Dr Boaler's critique of setting will appear in the next issue of Forum, out in November.