Failing students should repeat the last year of primary school, Tory leader David Cameron has proposed.
Mr Cameron said he would look at the system of holding pupils back
He said the move could form part of a "genuine schools revolution" improving literacy, numeracy and discipline.
Writing in the Sunday Telegraph, Mr Cameron calls for extra cash for heads if they take on disadvantaged pupils.
But Schools Minister Jim Knight said the proposals for pupils to repeat a year would only stigmatise children who needed extra help.
"They would also increase class sizes and make it impossible for teachers and parents to plan ahead," he said.
"We need to intervene early rather than holding kids back."
And a government spokeswoman said Mr Cameron's claim that 43% of children leave primary school unable to read, write and count properly was "quite wrong".
The Tory leader's intervention came as Schools Secretary Ed Balls urged schools to concentrate on the basics. He admitted government reforms had "not delivered for every child".
The Tories' proposals suggest the worst performers in year six would either have to catch up at summer classes or repeat the entire academic year.
Mr Cameron promised to "look carefully" at the measure, which is one of a host of proposals to be presented this week by the Tories' Public Services Improvement Policy Group.
But a government spokeswoman dismissed Mr Cameron's claim that 43% of children leave primary school unable to read, write and count properly as "quite wrong".
Re-sitting an academic year is already used in the US and some European countries.
Shadow schools secretary Michael Gove said it did work elsewhere. "It has ensured that levels of attainment in those basic skills - reading, writing and adding up - has improved."
But Jean Milham, head teacher of Morningside Primary School in Hackney, east London, told BBC Radio Five Live that the idea would mean children were branded as failures.
"Some of these children have got very special needs and they will never attain," she said.
And the general secretary of the National Association of Head Teachers, Mick Brookes, said the idea was flawed: "If you have 20% of children who are not reaching level 4 standards in primary schools, there would have to be a massive building programme to accommodate these children."
In his article, Mr Cameron said he wanted to give city academies - which some believe Gordon Brown is preparing to scale down - greater freedom to improve.
He said that under a Conservative government no more special schools would be closed down, and also promised "zero tolerance" of disruptive pupils.
A spokeswoman for the Department for Children, Schools and Families said: "It is quite wrong to suggest that a child not achieving level 4 'cannot read' or has 'failed the three Rs'.
"Level 3 in the national curriculum is the standard at which children have effective English and maths skills."
'Back to basics'
Meanwhile, Mr Balls admitted reforms were "not delivering for every child".
He told GMTV's Sunday Programme he would be writing to every headteacher in England to congratulate them on the results, but to say it was not enough.
"We must give a world-class education to every child in this country," he said.
"My message to teachers is I'm going to back you to do more to support the personal development of every child."
He said it was about "back to basics, standards in the classroom and not structural change".
And he would back teachers and heads so that they had the power to enforce discipline.
In a separate initiative at the start of the new school year, Mr Balls announced a £150m fund for building new kitchens in schools.