Clever children from poor families face being overtaken by less bright children from affluent homes, research suggests.
Social mobility has stabilised but not improved, research suggests
The findings are part of a study for the Sutton Trust which says UK social mobility has not improved since 1970.
It says rich children are catching up with poorer peers in developmental tests between ages three and five and will overtake them by the age of seven.
The government says it is too early to say what will happen to the young people the charity's report focuses on.
Researchers from the London School of Economics and the University of Surrey examined the development of children born in 2000 and 2001 to see if that was influenced by income.
They looked at test studies of children from various income groups at the ages of three and five.
The report said: "Children in the poorest fifth of households but in the brightest group drop from the 88th percentile on cognitive tests at age three to the 65th percentile at age five."
Meanwhile those from the richest households who were among the least able at three moved up from the 15th percentile to the 45th percentile by the age of five.
Report authors Dr Jo Blanden and Professor Stephen Machin conclude: "If this trend were to continue, the children from affluent backgrounds would be likely to overtake the poorer children in test scores by age seven".
They also said while 44% of young people from the richest 20% of households were awarded degrees in 2002, only 10% from the poorest 20% did so.
The report concludes: "Parental background continues to exert a significant influence on the academic progress of recent generations of children.
"Stark inequalities are emerging for today's children in early cognitive test scores - mirroring the gaps that existed and widened with age for children born 30 years previously."
Lee Elliott-Major, from the Sutton Trust, says the environment a child grows up in is all-important.
"Parental background is so dominant in terms of predicting and influencing people's future prospects," he said.
"It's about general aspirations, being in an environment that is conducive to talking about lots of different things, it's those sorts of very broad things."
The study also concludes that a narrowing of social mobility seen in the 1970s and 80s has now stabilised.
However, the report says the UK remains one of the worst among developed countries for social mobility, alongside the United States.
Minister for Children Beverley Hughes said: "As we look to the future we hope to see more evidence of our reforms making a real difference to people's lives," she said.
"This new research is based on the Millennium Cohort born in 2000-01. It's far too early to say what will happen to those young people over their lifetime.
"Those children have yet to enter Key Stages 2, 3 and 4, where overall standards are continuing to rise and poverty gaps have narrowed since 2003."