The age at which pupils start school should be raised to six or seven, a teachers' association suggests.
PAT says many children are not ready to start school at four or five
The Professional Association of Teachers (PAT) says many who begin formal schooling aged just four are not ready for an academic curriculum.
Deborah Lawson from PAT said it was vital that children should have more freedom to play in nurseries without being told what to do by adults.
The government said primary schools followed an age-appropriate curriculum.
Speaking at the PAT annual conference, in Harrogate, Ms Lawson said: "There is evidence that by starting school earlier, our children are not better off than those children who are starting later.
"As practitioners and parents we have all seen children who really were not ready to start school."
Time and space
She said toddlers developed social and language skills by playing in a way that was natural to them.
"We are not giving children sufficient time and space to learn for themselves - child initiated play activities.
"When they are playing outdoors, we talk about children letting off steam," she said.
"Actually they learn about the world around them by playing in the street or the park. They are learning those life skills that they need."
But some affluent parents want to know how their children are developing and request detailed reports from nursery staff, she said.
"Some parents like the worksheets because they are solid evidence that their child is progressing. They really want to see it," she said.
"Sometimes parents say 'Little Johnny is bringing back flash cards and my Billy isn't'. Actually, Billy isn't quite ready for it.
"When our children are going to be pressured academically and tested academically to destruction, why should we be putting that pressure on them?"
But a spokesman for the Department for Children, Schools and Families said: "The formal school starting age of five has served children well for decades and standards in our primary schools have never been higher.
"All the evidence - key stage results, international comparisons and Ofsted reports - make this clear.
"We want all children to make progress in literacy and numeracy at an early age, as these skills are critical to their ability to get the most out of learning later on.
"The first years of schooling focus on play-based activities in addition to formal learning - the curriculum is age-appropriate and we actively support teachers to adapt their teaching to the needs of children."