Public health groups urged the Government to take further action to prevent youngsters consuming junk food and sugary drinks, as the level of severe obesity hit a record high across England.
NHS Digital figures show that 22% of Year 6 pupils in Doncaster in 2017-18 were obese, of whomÂ 4.6% were severely obese.
Additionally, 13% of Year 6 children were overweight.
That means 35% of Doncaster's youngsters are unhealthily overweight when they finish primary school.
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Across England, 4.2% of 10 and 11-year-olds are severely obese, a record high.
Caroline Cerny, of the Obesity Health Alliance, a coalition of leading health charities, medical royal colleges and campaign groups, said "we can do something about this".
She explained: "The ever increasing number of children living with obesity is a clear reflection of the unhealthy wider environment that pushes us towards sugary and fatty food and drinks.
"We need to start with reducing the number of junk food adverts children see before a 9pm watershed, restrictions on junk food promotions in supermarkets and the food industry stepping up efforts to reduce sugar and fat from everyday foods."
Despite school meals getting healthier, the proportion of obese 10 and 11-year-olds in Year 6 has risen from 17% in 2013-14.
The figures are from the National Child Measurement Programme.
Each year officials measure the height and weight of more than one million children, in Reception and Year 6, to assess childhood obesity.
The Government works out obesity using the 1990 British growth reference chart, a large collection of statistics used to determine a child's body mass index (BMI). It defines a child as obese if their BMI is in the chart's 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 data shows that children often develop weight problems while at primary school.
In 2017-18, just 11% of Doncaster's children were obese in Reception.
Across England one in five pupils in Year 6 was obese. Children from the most disadvantaged backgrounds were more than twice as likely to be obese than those from the wealthiest areas.
Dr Max Davie, of the Royal College of Paediatrics and Child Health, said the figures were "totally unacceptable".
However, he said the Government "has already shown it is serious about tackling childhood obesity ... And I am reassured that these stats will begin moving in the right direction".
Public health minister Steve Brine said: "Obesity is a problem that has been decades in the making '“ one that will take significant effort across government, schools, families and wider society to address.
"We cannot expect to see a reversal in trends overnight '“ but we have been clear that we are willing to do whatever it takes to keep children healthy and well in this country.
'We have already removed tonnes of sugar from children's diets through the sugar tax, which has funded vital school sports and breakfast programmes, and this summer we announced the second chapter of our childhood obesity strategy with a series of bold plans to halve child obesity by 2030."