Britain's poorest children are almost three times as likely to be obese as the richest.
Children who are overweight or obese are far more likely to suffer serious illnesses like Type 2 diabetes, heart disease and cancer later in life.
Experts analysed data from a major study which tracked nearly 20,000 families across Britain. This allowed them to measure children’s sizes at age five and 11.
Their study found the link between relative poverty and childhood obesity was “stark”.
At age 5, poor children were almost twice as likely to be obese compared with their better off peers.
A total of 6.6% of children from families in the poorest fifth of the population were obese while the figure for the richest fifth was only 3.5%.
By the age of 11 the gap had widened, nearly tripling to 7.9% of the poorest fifth being obese, but just 2.9% of best off children.
The researchers examined many aspects of the children’s environment and health behaviours, such as mothers who smoked during pregnancy, how long she breastfed for and whether the child was introduced to solid food before the age of four months.
The study, which tracked nearly 20,000 UK families, also looked at whether the mother herself was overweight or obese.
Prof Yvonne Kelly, who led the study at University College London and the London School of Economics, said: “Intervening in the early years when the family environment has more profound influences on children’s healthy development has the potential to be particularly effective.”
The study also looked at frequency of sport or exercise, active play with a parent, hours spent watching TV or playing on a computer, journeys by bike and the time that children went to bed.
It also compared dietary habits such as whether the child skipped breakfast as well as their fruit and sweet drink consumption.
The study found that doing sport more than three times a week was important, as was an earlier bedtime and regular fruit consumption which were both linked to weight loss.
However, maternal smoking during pregnancy and a mother’s BMI were linked to weight gain among children.
Overall, the study found markers of unhealthy lifestyles could make a child 20% more likely to be obese.
Prof Kelly added: “The structural causes of socioeconomic inequalities have to be addressed along with tackling inherited obesity via lifestyle factors that tend to go with lower incomes.
“Early intervention with parents clearly has huge potential. And evidence from our work suggests that this should start before birth or even conception.”