The long-term development of the majority of children is being harmed by them being forced to start school aged four, according to a study.
Parents are coming under increasing pressure to enrol children early to make sure they get a place in the best schools, fuelling anxiety levels and damaging youngsters' self-esteem, it is claimed.
Latest Government figures show that almost 800,000 children in primary schools are aged four or younger, with around 80 per cent now entering before their fifth birthday.
Experts warned last night that the development risked damaging a generation of children who are "not being allowed to grow up". It comes a week afterBritain's children were branded the unhappiest in the West by a Unicef study.
Margaret Morrissey, of the National Confederation of Parent Teacher Associations, said: "Many small children aged four are barely out of their push chair when they are suddenly going to school. We have been assured that schooling at that age is informal and all about learning through play, but that is simply not the case.
"Thanks to inspectors, tests and league tables lessons even for four-year-olds are becoming very rigid.
Children can no longer wander off and doze on a bean-bag if they are tired – they are being pushed by an increasingly demanding curriculum."
The legal starting age for children is five. In the past, many parents could send them to school at three different points during the optional reception year, with many waiting until they were five or close to their birthday.
But a study by the Times Educational Supplement shows most schools now only admit children in September to maximise Government grants. If a pupil starts in January or April schools do not get funding for them.
According to the study, matters have got worse under Labour, with hundreds of schools denying parents the chance to choose to enrol children later in the year. An influential study by the National Foundation for Educational Research found that an early introduction to school can "increase anxiety and have a negative impact on children's self-esteem and motivation to learn".
Another study of 1,400 children in Glasgow found that boys who started at four-and-a-half were still at a disadvantage when they reached secondary school. Britain is almost alone in Europe for having a compulsory school age of five, let alone four. Most countries do not start children until six. And in most European countries, there is a strong kindergarten or nursery system where children learn in an informal atmosphere of play.
Mick Brookes, of the National Association of Head Teachers, said: "Why are we going increasingly against the trend which works so well on the continent? Most countries start compulsory schooling later than us yet by the age of 11 their children have overtaken ours."
A Education Department spokesman said: "Children do not have to start school until they are five though most areas offer admission to reception at four.
"Where children are not yet five, we encourage schools and local authorities to allow parents to defer their child's entry to a later point in the academic year, for example January or April."
By Graeme Paton
Source: The Telegraph