The percentage of overweight and obese 10 to 11-year-olds is the highest on record, official figures show.
Data from the National Child Measurement Programme for England shows obesity has risen in the last year, even for the youngest children being measured, aged four.
The data, from NHS Digital, shows that more than one in three (34.2%) children in year 6 (aged 10 to 11) were overweight or obese in 2015/16.
This is the highest on record and is up on 33.2% the previous year and 31.6% in 2006/07.
More than one in five children (22.1%) were either overweight or obese in reception year (aged four to five), up on 21.9% the previous year but under the 22.9% in 2006/07.
When it comes to obesity alone, 9.3% of children in reception class were obese in 2015/16, up from 9.1% the year before.
One in five (19.8%) of those in year 6 were obese, up from 19.1% the year before.
The National Child Measurement Programme measures the height and weight of more than a million children in England every year and is considered to be a robust analysis of children's weight.
A breakdown by regions shows that 5.1% of children in Richmond upon Thames in south-west London were obese in reception class in 2015/16, but the figure was 14.7% in Middlesbrough, which had the highest proportion.
In 2015/16, Richmond upon Thames also had the lowest obesity prevalence in year 6, with 11%, while Barking and Dagenham had the highest with 28.5%.
The Government has come under intense pressure over its childhood obesity plan, which campaigners say was heavily watered down in the months before publication.
Earlier this week, Channel 4's Dispatches claimed that curbs on junk food advertising and restrictions on unhealthy product placement in supermarkets were among measures cut from a draft of strategy before it was released in August.
A first draft of the plan leaked to the programme was claimed to contain a pledge to halve the number of overweight children by 2026.
This, it says, was changed to a pledge to "significantly reduce" the number of overweight children when the full strategy was published.
Among other proposals removed from the final strategy, according to Dispatches, were plans to force restaurants, cafes and takeaways to put calorie information on menus, forcing supermarkets to remove junk food from around check-outs and the end of aisles, and curbs to junk food advertising, including commercial breaks in and around popular Saturday night TV programmes.
Doctors, health campaigners and politicians reacted with fury when the plan was published in August, following months of delays.
In an interview with the Press Association, celebrity chef Jamie Oliver accused Prime Minister Theresa May of missing "a massive opportunity" to avert generations of obesity.
He said she had delivered none of the policy substance or the fanfare promised by former prime minister David Cameron.
The father of five said he made repeated efforts to help with the strategy - having been encouraged through several meetings with Mr Cameron - but felt his input was dismissed by Mrs May's team.
He said: "I have never met her, I have nothing against her, I wish her really, really well, I want her to be brilliant - but if you think that plan is amazing you would have put your name on it, and it wasn't.
"If you cared about it you would have celebrated it as a moment of pride.
"The launch we would have done with Cameron would have been a 'day', communicate it with the wider press and celebrities to say why this was going to be brilliant and celebrate it with the next generation.
"(Mrs May's strategy) was rubbish - none of that."
The NHS Digital data shows that obesity is more common among boys than girls (9.6% compared to 9% of girls in reception) and 21.7% of boys in year 6 compared to 17.9% of girls.
Obesity is also more common among black children than other ethnic groups, with 15.2% obese in reception class compared to 8.8% of white children or 9.6% of Asian children.
Across England, around 95% of children in reception and year 6 took part in the measurement programme.
Calculations by Cancer Research UK suggest the number of overweight or obese children leaving primary school (year 6) went from 178,868 in 2006/7 to 198,036 in 2015/16 - an increase of 19,168 over the last decade.
Alison Cox, Cancer Research UK's director of prevention, said: "Our nation has hit a devastating record high for childhood obesity.
"The trend over the last decade is showing no signs of slowing down, and this worrying news is something that could have been prevented with more government action.
"But the Government still has a chance to save lives. It has already recognised the influence of junk food marketing on children by banning junk food advertising during children's programmes.
"It's time now to stop this influential marketing before 9pm."
Professor Kevin Fenton, national director for Health and Wellbeing at Public Health England, said: "It is deeply worrying that more children are leaving primary school overweight or obese than ever before, and levels are increasing.
"The National Child Measurement Programme gives parents the opportunity to discuss their child's weight in confidence and receive expert advice on following a healthy diet and being active.
"Parents can support children by swapping sugary and fatty foods and drinks for healthier alternatives and keeping an eye on portion sizes."
The Obesity Health Alliance (OHA) - a coalition of more than 30 charities and groups including Action on Sugar, the British Heart Foundation, British Medical Association, Children's Food Trust, and Diabetes UK - said in a statement: "Another set of childhood obesity statistics and another bleak picture.
"Year upon year, we are faced with sobering figures that reveal an increasingly worrying trend - the number of obese and overweight children in the UK is not falling, and is in fact rising.
"The OHA is calling for immediate action on three fronts. Firstly, the Government's planned soft drinks levy must be passed without dilution next year.
"Secondly, Government must act without delay to introduce restrictions on junk food marketing at children - both online and on TV before the 9pm watershed.
"And thirdly, there should be ambitious targets for sugar reduction through Public Health England's voluntary sugar reduction programme - with widespread engagement and take-up by industry."
Shadow public health minister Sharon Hodgson said more needed to be done "to address the burgeoning crisis of childhood obesity here in the UK".
She added: "These findings should spur the Government to rethink their approach on childhood obesity and come clean about why they failed to do more to support the health of our nation's children when they published the childhood obesity plan in the summer."