Statistics from Public Health England show that 19 per cent of Year 6 pupils were declared obese, between April 2016 and March 2017, and four per cent severely obese.
And 14.4 per cent of Year 6 children were declared overweight.
That means on average 38 per cent of youngsters in Leeds are unhealthily overweight when they start secondary school.
And despite school meals getting healthier, the number of obese 10 and 11-year-olds in Year 6 has risen by nine per cent over the last five years.
The figures are from Public Health England’s National Child Measurement Programme.
Last financial year, 812 of Leeds’s reception age children were declared obese, while over the same time period 1,499 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.
She said: “They can see up to nine junk food adverts in just 30 minutes while watching their favourite shows.
“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
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.