It represents a “sharp increase” in the number who were obese at the age of seven, according to t he fifth Millennium Cohort Study (MCS) of more than 13,000 children born between 2000 and 2002 to be published by the Institute of Education tomorrow .
Dr Roxanne Connelly, who analysed the data, said the data indicates that children in the overweight category aged seven were “slowly creeping” in to the obese category by the time they were 11.
The analysis found the proportion of “children of the new century” who were classified as obese jumped from 13% at age seven to 20% at age 11.
Researchers also said there is a “clear link” between children’s weight at age 11 and their parents’ level of education.
Twenty-five per cent of boys and girls whose parents had no educational qualifications were obese compared to 15% of children who had at least one parent with a degree.
The data also found that children with overweight mothers were more likely to also be overweight, suggesting that children are copying their behaviour.
Dr Connelly said: “One of the key issues we now need to focus on is why there was such a sharp increase in overweight and obesity among the MCS children between ages seven and 11.
“The number who were either overweight or obese rose from 25% at age seven to 35% at age 11.
“One thing we did find was that a lot of children who were overweight at age seven were slowly becoming obese so it could be a creeping problem.
“Only a small number of overweight children aged seven moved down to a healthy weight.”
She added: “These findings highlight the value of the Millennium Cohort Study for addressing issues relating to child health and, in particular, for advancing our understanding of the ‘obesity epidemic’.”
Children who are overweight or obese face an increased risk of many health problems, including asthma, cardiovascular disease and type 2 diabetes and is also associated with psychological problems such as low self-esteem and depression.
Those who were obese at age 11 were less likely than other children to be “completely happy” with the way they looked, the researchers found.
They were also slightly more likely to say they were “not happy at all” with their appearance.
The study has so far collected data from the 13,287 boys and girls and their families at the ages of nine months and three, five, seven and 11 years to analyse their health, schooling and development as well as their parents’ employment and education.
The latest survey of 11-year-olds was carried out by Ipsos MORI between January 2012 and February 2013.
Overall, almost half (49%) of the millennium children were classified as overweight or obese in at least one of the four latest surveys.
Dr Ann Hoskins, a director at Public Health England, said: “It is deeply concerning that there is a virtual doubling of obesity rates from reception to the end of primary school, as also shown in our own data, and that it is particularly worse for children from low income households.
“Parents and carers can help their children maintain a healthy weight by following a balanced diet, ensuring portion sizes are not too large and avoiding sugary drinks and sugary or fatty snacks.
“Children also need a minimum of 60 minutes of moderate to vigorous exercise each day, which can be achieved in one session or through shorter bursts of 10 minute activity.
“PHE are working to help local authorities tackle the environmental causes of obesity.
“Our guidance on the regulation of fast food outlets provides local authorities with recommendations and case studies such as using 400 metre fast food outlet exclusion zones in school areas.
“We are also working with schools to promote whole school approaches that help pupils lead healthier lives.”