Nearly 40% of children aged 10 or 11 could be obese by 2024, according to Public Health England.
Last year, 34.3% of children in that age group were overweight or obese, which is up from 31.7% recorded in 2006/07.
But according to a forecast in Public Health England’s (PHE) Health Profile for the country, published on Wednesday, this figure could be between 33.4% and 38.1% in just five years.
This could mean nearly four in ten children will be obese by the time they leave primary school.
Professor John Newton, the PHE's director of health improvement, said it was "important not to interpret this trend as a sign that what we're doing at the moment isn't working".
However, with the Government already setting an "ambitious target" to halve child obesity rates by 2030, he added: "It is a reminder we need to redouble our efforts on childhood obesity.
"The target is certainly an ambitious target, everybody would agree with that.
"Given the importance of child obesity it is important to have an ambitious target."
He added there was also "a lot" of variation in child obesity rates around the country, linked to deprivation.
Professor Newton said: "The question is, why does it vary so much, and that is because the conditions in which children live around the country, vary hugely.
"It goes back to a strong public health message.
"We need to build healthy communities and give every child the chance to grow up in a healthy community."
Professor Newton said child obesity levels were of particular concern because they lead to unhealthy adults, diabetes, heart disease and other conditions.
Since 2007, there has been an upward trend in adult obesity and it has been rising faster than previously forecasted.
If this continues, between 26.6% and 33.9% of adults could be obese by 2024.