More than one in five children leaving primary school in Sheffield are now obese, with one in 20 classed as severely obese.
New figures reveal the proportion of the city's children who are battling weight problems has reached its highest ever level, laying bare the scale of the brewing health 'crisis'.
In Sheffield, statistics published this month by NHS Digital show 21.2 per cent of pupils in year six - the last year before starting secondary school - are now obese.
Those figures for 2016/17 are 0.5 percentage points higher than the previous year, and well above the national average of 20 per cent.
More than one in three (35.6 per cent) year 6 pupils in the city are either overweight or obese, and 4.97 per cent are severely obese - compared with a national average of 4.07 per cent.
The figures, from the National Child Measurement Programme, were highlighted by the founder of a Sheffield charity who has made it her crusade to improve support across the country for young people battling complex weight problems, which are often linked to mental health.
Kath Sharman, of SHINE Health Academy, said: "I'm not shocked by these figures. I'm just delighted they've started recording the number of children who are severely obese so we can get a real sense of this crisis.
"We know there's a big increase in the proportion of overweight children between reception age and year six but we don't know precisely when that's happening, why it's happening or what can be done to prevent it.
"School nurses used to measure children every year, meaning they could act very quickly if a child was putting on too much weight. Now by the time a problem's identified a lot of the damage has already been done.
"We need to be measuring children more regularly and investing more time and money to tackle weight problems early if we're going to reduce adult obesity."
Greg Fell, director of public health at Sheffield Council, said: "We are aware that child obesity rates are rising nationally, including in Sheffield. To tackle this growing problem, we need stronger advocacy from government to support us in our efforts locally."
He claimed tighter restrictions on marketing junk food to children, mandatory food labelling, a cap on portion sizes and sugar levels of certain foods, and regulations to reduce price promotions on unhealthy food were among the measures needed.
In Sheffield, he said a new food and well-being strategy, including plans to work with schools to cut the amount of sugar young people consume, would soon be announced.
He also highlighted the city's Child Weight Management Service, which he said was helping overweight children get healthy; Start Well Sheffield, which supports parents with young children to improve their diet and lead a more active lifestyle; and the Move More initiative, which is part of Sheffield's goal to become the country's most active city by 2020.
Ms Sharman has launched a petition calling on the Government to fund specialist support for children across the country with severe or morbid obesity.
She hopes to get at least 2,000 signatures so young people who are battling weight problems or have overcome them can get their voices heard in parliament.
* You can sign the petition at https://petition.parliament.uk/petitions/205447.