Children from the most deprived backgrounds in Peterborough are more than twice as likely to be obese than those from the most affluent, the latest data has revealed.
Statistics from Public Health England show that 23% of Year 6 pupils from the most deprived section of society are obese.
That's more than double the amount of youngsters from the least deprived backgrounds, where 10% are obese.
Each year the National Child Measurement Programme (NCMP) measures the height and weight of more than one million children, aged between four and five and 10 and 11, to assess childhood obesity.
The prevalence among deprived youngsters was calculated over a four year period from between April 2012 to March 2013 and the last financial year.
In Peterborough from 2016 to 2017 almost one in four children finishing primary school were classified as obese, one of the highest rates in England.
On top of that 5% of Year 6 children were declared severely obese, and a further 14.2% overweight.
That means on average 42% of Peterborough's youngsters are unhealthily overweight when they start secondary school.
The government works out obesity using the 1990 British growth reference chart, a large collection of statistics which work out a child's body mass index (BMI).
It defines a child as obese if its BMI is in the top 5%, and overweight if they are in the top 15%.
Children's BMI is measured differently to adults, and is calculated using age and gender as well as height and weight.
Obesity can lead to heart problems and type 2 diabetes later in life, as well as psychological issues such as low self esteem and depression.
The number of obese 10 and 11-year- olds in Year 6 has risen by 35% over the last five years.
The data consistently shows that children develop weight problems while at primary school.
Last financial year 231 of Peterborough's Reception children were declared obese, while over the same time period 524 in Year 6 hit the same unhealthy weight.
The obesity rate for 10 to 11-year- olds is more than double that of four to five-year- olds.
Caroline Cerny, lead for the Obesity Health Alliance, a coalition of more than 40 organisations that have joined together to prevent obesity related ill health, described the figures as "startling".
She said: "These latest figures show childhood obesity has risen over the last two years, with more children overweight or obese when they leave primary school than when they start.
"Children with obesity are five times more likely to become obese adults, putting them at risk of a number of devastating conditions such as type 2 diabetes, cancer, heart and liver disease.
"We've seen a certain amount of progress from government, including the implementation of the soft drinks levy from April this year. But far more needs to be done."
Miss Cerny called for junk food adverts to be banned while youngsters are watching television.
"They can see up to nine junk food adverts in just 30 minutes while watching their favourite shows," she explained.
"Government must take decisive action to stop children being bombarded with adverts for junk food by introducing a 9pm watershed."
Commenting on the figures, public health minister Steve Brine said: "These numbers confirm what we know and exactly why we are taking action.
"We are delivering the most ambitious childhood obesity plan in the world taxing sugary drinks, helping children to exercise more, funding more research and cutting sugar and calories in food before it hits shelves and plates.
"We are confident our world-leading plan will change this trend but it's very early days tackling a problem decades in the making, and we have not ruled out further action if the right results are not seen."
Last August the government announced a new plan to tackle childhood obesity. Public Health England will look at unhealthy meals which children are eating, such as pizzas, burgers and ready meals, and come up with a comprehensive calorie reduction programme later this year.
In April the sugar tax will come into effect, applying an extra levy on soft drinks with a sugar content of more than 5g per 100ml, such as Coke, Red Bull and Dr Pepper.