More than 1.6 million children who started secondary school in the past decade were overweight or obese, new calculations show.
Between 2006/07 and 2014/15, there were 1,654,894 children in England who started Year 7 with an unhealthy weight, according to Cancer Research UK.
The alarming figure foretells a future of ill-health for the nation, the charity says.
It added that obese children were about five times more likely to become obese adults.
The charity has called on the Government to press ahead with measures set out in its Childhood Obesity Plan, which was published in the summer.
Many leading health organisations criticised the plan because it did not contain new curbs on junk food advertising.
The plan had an emphasis on greater physical activity in schools and a voluntary scheme for the food industry to reformulate popular children's products to reduce sugar.
Also central to the document is the Government's sugar tax on soft drinks.
Alison Cox, Cancer Research UK's director of prevention, said: "It's concerning to know that so many children start secondary school – formative years in a child's life – carrying too much weight.
"We must give children the best chance for a healthy future. Measures like the sugary drinks tax can make a difference and the Government must press ahead with this vital measure."
The charity said that carrying too much weight increased the risk of cancer as well as other diseases.
Being overweight or obese contributes to about 18,100 cases of cancer every year in the UK.
Having an unhealthy weight is linked to 13 types of cancer including bowel, breast and pancreatic.
Meanwhile, new figures from the Obesity Health Alliance show an apparent "weight gap" between poorest and wealthiest primary-school aged boys living in England.
The Alliance, which is made up of leading health organisations and charities, said that 60% of the most deprived boys aged 5-11 are predicted to be overweight or obese by 2020, compared with 16% of boys in the most affluent group.
Meanwhile, an average of one in five girls is predicted to be obese or overweight by 2020 - with no difference between the richest and poorest girls.
Robin Ireland, chief executive at Health Equalities Group and member of the Obesity Health Alliance, said: "From a young age, children are developing a taste for high sugar, salt and fatty foods that is difficult to break once established and as a nation, we all have a responsibility to help shape children's diets.
"Sugary drink consumption levels tend to be highest among the most disadvantaged children who are hit hardest by obesity and tooth decay. The health gains from the soft drinks industry levy will be biggest for our most deprived children."
Chris Askew, chief executive at the charity Diabetes UK, added: "Obesity is a major risk factor in developing type 2 diabetes, and type 2 accounts for nine out of 10 diabetes cases.
"Treating diabetes and its complications already costs the health service £10 billion a year and the rising cost is placing huge pressure on the NHS. Not taking action now will result in the NHS forking out monumental amounts of money for largely preventable conditions."
Professor Russell Viner, officer for health promotion for the Royal College of Paediatrics and Child Health, said: "Obesity blights a childhood and damages adult life, raising the risk of serious complications such as type 2 diabetes and breathing problems – conditions we are seeing much earlier in childhood.
"We need to make healthier food the easier, cheaper choice by introducing advertising restrictions before the 9pm watershed, and testing the impact taxation has on foods high in salt, sugar and fat."
A Department of Health spokeswoman said: "We want all children to have a healthy future and are confident our world-leading plan to reduce childhood obesity will make a real difference.
"UK restrictions on advertising are already some of the toughest in the world and we have been clear that we expect the food industry to take strong steps to make their food healthier. Our plan is the start of a conversation and we haven't ruled out further measures if we do not see the progress we need."
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "It is deeply worrying that children are twice as likely to be overweight or obese if they live in a deprived area.
"The PHE sugar and calorie reduction programme will remove 20% of sugar from the products children eat most as part of a wide-ranging programme of work to tackle obesity."