As youngsters prepare to start school next week, Cancer Research UK said that one in three will finish primary school either overweight or obese.
New figures from the charity show that every year 57,100 children who started primary school in England at a healthy weight end up obese or overweight by the time they leave.
When youngsters start primary school one in five are overweight or obese, but when they leave that figure has risen to one in three, the charity said.
Cancer Research UK has transformed a shop front in east London into an 'XL school uniform shop' to highlight the growing obesity epidemic.
The charity has accused the Government of "failing" youngsters with its childhood obesity plan.
When the document was published two weeks ago, leading health organisations said that the plan did not go far enough to help stem the number of children who are overweight or obese.
Curbs on junk food advertising did not form part of the document, despite repeated calls from health experts.
Alison Cox, Cancer Research UK's director of prevention, said: "The Government has failed children. More than 57,000 children will become overweight or obese during primary school each year in England, and the Government had a chance to prevent this.
"The childhood obesity plan is simply not up to the task of tackling children's obesity. Instead, the next generation faces a future of ill health, shortened lives, and an overstretched NHS.
"It will take more than encouraging exercise and a sugar tax to tackle the obesity epidemic. The Government has already recognised the influence of junk food marketing on children's health by banning junk food advertising during children's programmes - it's time to close the loop hole during family viewing time.
"Young waistlines have been expanding steadily over the last two decades. With so many overweight and obese children in England, we are seeing a greater need for larger school uniforms. And it's a shame the Government has missed an opportunity to save lives."
