Half of children have not attained their expected level of development when they move on from reception to their first year of school, new figures suggest.
Only 52% of five-year-olds in England have reached a "good" level of development, the data shows.
As few as 28% of children have reached this level in some parts of the country, experts from University College London's Institute of Health Equity said.
Professor Sir Michael Marmot, director of the Institute, said the figures show that we "continue to fail our children".
Sir Michael published a review of health inequalities in England in 2010 and has continued to monitor the situation. Among the social indicators measured which contribute towards health inequalities is assessing the level of development among children.
He said: "We continue to fail our children. How can this still be happening? For three years the Institute of Health Equity has published evidence showing we are failing our children. It is unacceptable that only half of our five years olds are achieving a good level of development.
"(So far) we have seen that the more deprived the area, the worse the early childhood development.
"In 2011 in England as a whole only 59% of children have a good level of development age five. Now only 52% of children age five have a good level of development.
"When we first looked at these figures we assumed there must be something wrong with the measurement. How can it be the case in England, one of the richest countries in the world with our long history of being a brainy country, that only 52% of children can have a good level of development?"
He said that when compared to other western countries England is "bumping along the bottom" in the rankings for early childhood development. "We are doing really badly," he added.
"One strategy for reducing these avoidable inequalities in early childhood development is to reduce deprivation, improve living standards for families with children.
"We know that can be done through the fiscal system, through taxes and transfers."
The figures show that some local authorities are performing significantly better than others.
In the best performing area, Greenwich in south east London, 69% of children age five had reached their expected level of development.
But in Leicester just 27.7% attained this.
"However good or bad the school is, which will of course have an impact on children's performance, children's level of development as they enter the school is a potent predictor," Sir Michael said.
"That measure gets worse and worse the lower down the social hierarchy they are."
He said he believes that poor performance relates to levels of deprivation and the quality of services to support children and their parents before they start school.
"In looking at children's development you have got to look at children, parenting, the circumstances in which parenting takes place - the conditions of parents' lives - and the general social context.
"Of course, some children will do better than others because it's in their genes or things of that nature but we know the quality of parenting is crucial. We know input from parents or other carers on talking to children or reading to children, playing with children, singing with children and warmth - emotion, cuddling, loving - are all vital to children's development.
"Then, of course, the context of parents' lives - the degrees of inequality, what's happening in communities, opportunities for work and so on."
He added that closing Sure Start centres for children does not help youngsters improve early development. "Good services make a difference and that's the message around this variation."
Sir Michael also expressed concerns that ministers have said the assessment for children's development - which looks at a number of areas of development, including communication and language skills; physical development; personal, social and emotional development; literacy and maths skills among others - will no longer be mandatory.
"It has been the case until now that all over the country there are measures of early childhood development at the end of reception for every child age five.
"The Department for Education has said that this will no longer be mandatory, I think that's a real shame. It has been brilliant that we've had these measures so we can actually check what's going on."
A Department for Education spokesman said: "No child should start school behind their peers. This is why our plan for education is providing flexible, affordable and good quality childcare.
"We are raising the status and quality of the early years workforce by introducing rigorous new qualifications so practitioners are highly skilled and can help all children reach the expected level of development. Furthermore, we are investing £50 million from April next year to extend the pupil premium to the early years."
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