Programmes using basic toys could boost educational achievement of 200 million developing world children not reaching their potential, a report says.
The researchers said some children had not used a pen before school
Poverty, a lack of stimulation and malnourishment leaves millions unable to benefit from schooling even if it is available, the Lancet study says.
The kind of early play taken for granted in the West can boost development, the UK-led team says.
It wants governments, international agencies and charities to act urgently.
The research has been carried out by an international team of experts led by Professor Sally McGregor of University College London's Institute of Child Health.
It seeks to fathom the scale of educational underachievement in the developing world.
Using World Health Organisation data on stunted growth and information on severe poverty, from a series of long-term studies in seven developing countries, her team calculated that some 200 million children were held back intellectually.
There was already convincing evidence that extreme poverty and stunted growth led to backward educational achievement, she said.
But despite the scale of the problem, simple steps could be taken at an early stage to improve children's life chances.
Without such steps the aim of providing all children with access to primary education would be meaningless unless their development is tackled before school begins.
It was well known that the ability of children to do well at school depended, to some extent, on their IQ level when they arrived in reception class, the professor said.
She had seen villages in sub-Saharan African nations such as Kenya where children had never picked up a pen or seen a picture in a book by the time they reached school.
She added that in many areas of the developing world, it was thought that play was something that children did alone rather than with adults to learn.
She said: "This is a vast problem. But with political will it is a solvable problem.
"Investing in programs in children under the age of five will be both more effective and far cheaper than leaving it till later.
"Bluntly, we can make these children more intelligent and help them benefit more from education. We can show the benefits of these programs last into adulthood."
Prof McGregor added: "Research over decades in Jamaica and other countries has shown that women with only primary school-level education and a few home made toys can be trained to make a significant difference in the education, intelligence and mental health disadvantaged children."
Such projects, encouraging learning through play, had led to children's attaining higher IQs and getting better reading skills, she said.
But she warned: "The Millennium Goal of universal primary education for all cannot be met unless these children's poor development is tackled."
"We are not going to break the poverty cycle if we don't get the children in a state where they can benefit from school.
"We really want to get the attention of international agencies, charities and national governments to show them what a difference we can make by doing something very simple," she added.
The report came as Chancellor Gordon Brown repeated his calls for free education to be provided for every child in the world by 2010.
The Lancet paper is one of a series of three looking at underachievement in the developing world.