The family background of children born at the turn of the century is still the most powerful factor in determining their development in school, the latest evidence from a long-term study has suggested.
The fifth Millennium Cohort Study (MCS), published by the Institute of Education today, also found that four times as many children experienced a crucial change in their family structure by the age of 11 than 45 years ago.
The director of the UK study said it was a concern that the performance of boys and girls starting secondary school continued to be affected by their family background and raised fears that attempts to offer equal opportunities to children were not reaching the most at risk.
More than 13,000 11-year-old boys and girls were surveyed in the latest stage of the study, believed to be the largest of its kind, which has followed them since their births.
It found children of the highest-qualified parents continue to significantly outperform those whose parents are unemployed or have manual labour jobs.
The data also revealed t he number of families headed by two natural parents, either married or co-habiting, dropped from 85% when the children were aged nine months, to 61% at 11 years old.
Thirty-five per cent were affected by a significant change in their parents' relationship status, including divorce or a step-parent moving in to the family home, and were more than twice as likely to develop behavioural problems by the age of 11 than children living with two natural parents.
Professor Lucinda Platt, the director of the latest MCS study, said: "This is a huge study which highlights how much children's lives have changed over the past few decades.
"There are concerns that family background including parental education and class still matters for this generation of children, it matters in terms of cognitive development and their learning.
"It's an important issue and one we could perhaps do better on. Our understanding of social mobility is based on studies of previous generations so this tells us a lot about the children growing up now.
"We have a huge amount of data which is why this study really scratches the surface of the lives of children born at the beginning of the century.
"It's allowed us to to look at whether there are issues around inequality of opportunity."
Over the five studies, more than half of the children (53%) were classified as living in poverty at some point, with one in six (17%) brought up in persistently poor families.
Persistent poverty was particularly high among children who were in a workless family (50%) or were being brought up by a lone parent (30%).
Professor Platt added: "Our findings are concerning because poverty is undoubtedly bad for children.
"It can have a negative effect on their educational attainment, health and behaviour in childhood, and can have adverse consequences in adulthood.
"Long durations of poverty put children at particular risk of poorer outcomes during their school years and in later life."
But she added that despite the very different lifestyles of millennium children compared to previous generations, thee-quarters (75%) said they were "completely happy" with their families while half were "completely happy" with themselves.
Only one in 10 children said they did not like school and more than two-thirds of children (70%) had a mix of friends from ethnic minorities.
Nearly three-quarters (72%) of the 11-year-olds had their own mobile phone, a third of which had access to the internet, while just under a fifth (17%) spent three or more hours a day watching television or using a computer.
Professor Platt said: "In the study, we found a lot of differences between children's lives and the results were varied but what was most striking is that overall the children seemed happy and many evaluated their lives quite positively."
The latest survey of 11-year-olds was carried out by Ipsos MORI between January 2012 and February 2013.
The 13, 287 children born between 2000 and 2002 have completed five surveys throughout their lives, at nine months-old, three, five, seven and 11-years-old.