Plans to force all children to start school at four will leave many youngsters "stressed and upset", according to academics.
They said pushing young children into formal classes too early would fail to improve their education - and may seriously harm their long-term development.
It will particularly affect summer-born children who often struggle to keep up with older classmates, it was claimed.
The comments come amid a growing revolt against the plans to abolish staggered entry to primary school.
At the moment, most children start reception classes in the September after their fourth birthday.
But parents of children born in July or August can enrol them later - often in January - giving youngsters longer to develop at home.
A Government-backed review of primary education has recommended that all children begin at the same time.
Sir Jim Rose, former director of inspections at Ofsted, said they needed more opportunity to improve their communication, language and literacy amid fears summer-born children under-perform in tests at the age of 11 and 14.
In an interim report, he said that they should be given the chance to attend part-time and insisted the curriculum should remain "play-based".
But academics fear the youngest are still being pushed too quickly.
Much of the rest of Europe delays formal education until six. Children in Scandinavian countries do not start formal lessons until seven but often outperform those in UK schools.
David Whitebread, senior lecturer in the psychology of education at Cambridge University, said in the short-term children often did well, although that didn't mean it was "the right thing to be doing".
He told the Times Educational Supplement(PLEASE LEAVE THIS IN): "You could teach four-year-olds the flags of the world, and they would do better at it than if they were not taught them at all. But it wouldn't make any sense to do so. It is a big mistake. Children learning to jump through hoops are not really learning, not understanding, and a significant proportion will be put off because they are being asked to do something they find difficult, that stresses them out and makes them upset."
Academics also claim summer-born children are up to a fifth more likely to be misdiagnosed as suffering special needs because they struggle to keep up with classmates. Researchers from Oxford, Nottingham and the London universities tracked 3,000 children and found a third of autumn-born children were identified as having special needs compared to 46 per cent with birthdays in the summer.
Richard House, senior lecturer in the school of human and life sciences at Roehampton University, said "pressure from local authority targets" meant many reception teachers subjected four-year-olds to an inappropriately academic curriculum.
A spokesman for the Department for Children, Schools and Families said: "The consultation into Sir Jim Rose's review does not finish until the end of February and he will consider carefully all responses."
(By Graeme Paton, Education Editor Last Updated: 6:56PM GMT 16 Jan 2009)