The sight of four-year-olds trotting off to primary school at 9 o'clock in the morning and returning home at 3pm is becoming a common one as local authorities all over the country expand to meet parental demand and government wishes. But do we want children to be thrown into large and relatively formal classrooms for the whole day at so tender an age? Lucy Hodges asks if such treatment will produce the learning we need, and finds that the experts are deeply divided.

We British are unusual in starting our children in compulsory education at five. On the Continent most countries start formal education at age six, once children have acquired the social skills needed for group learning.

Some Scandinavian countries begin education even later - Finnish, Danish and Swedish infants start school at seven. In America the compulsory starting age is six. Many of these countries do no worse than Britain in international league tables, and a few do better.

What is worrying early years experts is the new trend in Britain for children to begin school at age four. "That is too early to start teaching most children to read and write," says Wendy Scott, who chairs the British Association of Early Childhood Education and is an Ofsted inspector for early years education. "It just turns them off learning."

Margaret Lochrie, chief executive of the Pre-School Learning Alliance, echoes her worries. "You have to ask whether reception classes in primary schools are good settings for children this young," she says. "In many cases they're not. We know that many children suffer because of the incidence of bed-wetting, malaise, unwillingness to attend school and general unhappiness."

Research carried out by the National Foundation for Educational Research supports this, showing that children starting school close to their fourth birthday do not do as well as children of the same age starting school later. That research, which has just been confirmed by another similar NFER survey, raises important questions about the ability of school reception classes to cater for the needs of younger four-year-olds, according to its authors Caroline Sharp and Dougal Hutchison.

"Although some education authorities and schools have made strenuous efforts to improve the provision in reception classes, funding to provide teacher training, improve teacher-pupil ratios and enhance buildings and equipment has been slower to materialise," they say.

In other words, many of the new reception classes are failing four-year- olds. They are too large (sometimes containing well over 30 children) with inadequate numbers of trained staff. Moreover, because of the advent of the national curriculum, they are engaging in quite formal learning which may not suit pupils that age. That contrasts with nursery schools which have traditionally emphasised exploratory and practical work - learning through play - and where the pupil-teacher ratios are more favourable.

But not all research points to the same conclusions. At Durham University, Peter Tymms, reader in education, says it is not at all certain that an early start is bad for many children. He was commissioned by the Schools Curriculum and Assessment Authority to examine the literature on the subject and found the evidence was contradictory.

"I have been struck by the enormous progress that children make during their first year at primary school regardless of the age at which they come in," says Mr Tymms, who has been conducting a big survey of primary schools. "We don't have any solid evidence for what is the best way forward. It's a complicated subject and we don't have good information on it."

Effective reception classes can provide a good environment for very young children, he says. Accusing some of the early years experts of scaremongering about class size, he says it is rare to find a reception class with 35 children being looked after by only one adult. The teacher will usually have a helper, he claims.

Nonetheless, another feature of the English and Welsh education system which worries some experts is the insistence on children beginning school at a time tied strictly to their chronological age. It is done in the name of equality but takes no account of individual children's needs or parental choice, according to its critics. The practice contrasts with that on the Continent. It also contrasts with the American system where parents are free to hold their children back a year - and often do - on the grounds that they will do better that way.

A newly published study carried out by Sig Prais, of the National Institute for Economic and Social Research, compared a group of Swiss and English nine and 10-year-olds. He wanted to observe what effect the Continental practice of holding back slow-developers for a year longer in kindergarten had, compared with the English practice of school entry linked to age. (The Continental practice is designed to achieve a narrower range of ability in each class and thereby make teaching easier.)

The effect was clear. The slow-developers who had been held back performed close to the average of the class in which they were placed. The variation seen in English pupils' mathematical attainments in a given class was reduced by about half.

Sig Prais - and other academics - admire the Continental countries for the emphasis they put on early years education as a distinct phase in which children are given different treatment to match their needs. Peter Moss, a researcher at the Thomas Coram Research Institute, says: "On the Continent, children are in a strong model of early years education where you can develop continuity, rather than shooting them about like pinballs from one place to another."

It is against this background that Wendy Scott and two colleagues have drawn up what they are calling "An early years agenda for the new millennium", which they have sent to David Blunkett, Secretary of State for Education. They want to see education for three to six-year-olds treated as a distinct phase and shielded from the demands of the national curriculum until the beginning of the academic year in which children are six.

Fragmentation is a feature of the current British scene - hastened by the last Government's nursery voucher scheme for four-year-olds (now abandoned). Government funds were to be taken away from local authorities and handed back for vouchers. Therefore - to recoup the lost money - authorities laid on reception classes.

In a number of areas the current local policy is that all four-year-olds leave nurseries and enter reception classes. The fragmentation can mean under-fives switching between three environments in 18 months. Toddlers, aged three, may start out in pre-schools for a few terms, move on to nursery for another three terms and finally into reception classes at the age of four.

Some experts claim such fragmentation gives children a poor start in life. The new government's policy of providing places for all four-year- olds, whether they are in private or local authority funded pre-schools, nurseries or reception classes will not reduce the trend to more reception classes. No central figures exist on the number of four-year-olds currently in such classes but a study of 57 local education authorities by the Pre- School Learning Alliance has found the majority were expecting to provide reception classes for more than 96 per cent of eligible four-year-olds.

Martin Woodhead, senior lecturer in child development at the Open University, points out that these changes in arrangements for children to start school amount to a small revolution. They have taken place with almost no debate in Parliament or elsewhere. He says: "The precise educational rationale for the school environment being offered to four-year-old children has either been given inadequate attention or overlooked altogether."