It comes at a time when growing numbers of parents are choosing to keep their children in nurseries rather than sending them off to primary school after their fourth birthdays. More and more education authorities have been providing for four-year-olds in their reception classes - a trend boosted by the last government's introduction of vouchers.By laying on extra classes for four-year-olds, local authorities could recoup the money being switched to vouchers.
Caroline Sharp and Dougal Hutchinson, who conducted the study for the NFER, say the length of time a young child is in school has some clear effects. Autumn-borns who have been at school longer appear to do better, and summer-borns who have been at school for only six terms do least well. But, neither the spring nor the summer-born group appears to derive an additional benefit from the full nine terms at school.
The researchers say: "It has raised some important questions about the ability of school reception classes to cater for the needs of younger four-year-olds.
"Although some education authorities and schools have made strenuous efforts to improve the provision in reception classes, funding to provide teaching training, improve teacher-pupil ratios and enhance buildings and equipment has been slower to materialise."
Margaret Edgington, a former nursery head and now an early years consultant, is receiving an increasing number of calls from parents who are fearful about their children moving to primary schools when they are just four. She claims they are being "blackmailed" by authorities chasing the money that a new reception class pupil will bring in. They are told that if they don't send their child to the reception class in September, the school won't be able to hold a place for the January term when the child is four-and-a-half. Indeed, if the school is popular, the parents may miss the boat altogether. "It is a complicated issue and one that many of us have been worried about for some time," says Ms Edgington. Research done over a number of years shows that putting young four-year-olds into large reception classes can switch children off from schooling, she says.
Wendy Scott, chairman of the British Association of Early Childhood Education, says she has argued consistently that early entry is not helpful to anyone "except the school budgets".
In Bristol, some parents have complained of feeling coerced into sending their children to reception classes. Barbara Mackay says she prefers to keep her four-year-old son for a bit longer at St Werburgh's Park nursery because of its favourable pupil-teacher ratio and "because it's so good. There's a lot of thought put into each activity," she says. "My son is happy there. He barges in here every morning and wants to be the first one in the class."
Alexander Fleming's daughter had her fourth birthday a month ago. He and his wife felt she was too young to start school this term - though they were happy for her to start in January - and were startled to be told three weeks before the end of the summer term that she would not be allowed to stay at the nursery and would have to go to a reception class with 36 pupils.
All the information they had received to that point had been by word of mouth - something which prompted Mr Fleming (not his real name) to write letters to Bristol education authority and the two headteachers. It took six weeks for the education authority to reply, he says. But his intervention worked. His daughter will start at the reception class in January.
Mr Fleming says he is surprised at the authority's inefficiency and lack of concern for the children's needs and parents' wishes - until his letter hit the education authority's offices. "It was a terrible period," he says.
Bristol confirms its policy is to move all four-year-olds to reception classes, but says it does believe in parental choice. In exceptional cases,parents can keep their children in nursery schools, although choice of primary school cannot then be guaranteed.
Jane Lingham, a parent and a secondary school teacher, claims the head of the local primary school in Luton, which has a new four-plus unit this term, was trying to pressurise parents to send their children to his school this September. She was expecting her son, who will be five at Christmas, to start school at Easter.
"People were concerned they wouldn't be able to get their children into the primary school at all if they didn't opt for September entry," she says.
"I took a step back and thought `What do I want for my son? I want him to stay where he is'. " She has therefore kept him at the nursery and will seek a reception school place for him when she thinks he is ready for it. But the experience has left her surprised at the extent to which parental choice is ignored and about the lack of information provided by the authority.
A spokesman for Luton Borough Council said: "Our policy is to discourage schools from placing any pressure on a parent to move their child from a nursery to a four-plus reception unit unless the child is happy and ready to make the transfer."
In all these cases parents managed to reach a compromise for their children.
But Naomi Wright (not her real name), who lives in the south of England, failed completely. Her daughter, aged just four, started in reception class this week. Had she been eight days younger she would be starting next September.
Mrs Wright, who says her daughter is an immature four-year-old, wrote to Education Secretary David Blunkett as well as to education organisations and her authority - all to no avail.
The county told her that her daughter would still have to take her national tests at the correct chronological age which would mean taking them a year before she was ready, and she would also have to leave school at the correct chronological age and therefore miss a year. "I have no choice whatsover," she says.
How do season of birth and length of schooling affect children's attainment at key stage 1? A question revisited. By Caroline Sharp, senior research officer, and Dougal Hutchinson, chief statistician, at the National Foundation for Educational Research