Devon is one of a few areas in the country expected to see an improvement in the level of overweight or obese children, new research suggests.
The Royal Society for Public Health, which carried out the research, said the nation is "failing the test on childhood obesity" as it warned children in the most deprived areas will be worst affected.
Based on the Government's national child measurement programme, the RSPH analysed data from 2009-10 to 2023-24 to predict the level of overweight or obese year 6 children across England.
In Devon, the researchers found 31.6% of year 6 children were overweight in 2023-24 and projected this to fall to 29.8% in 2034-35.
Across England, 90% of areas are set to see the level of overweight children worsen.
Nationally, 36% of year 6 students were already overweight or obese in 2023-24. The research predicts this will jump to 41% in 2034-35.
William Roberts, RSPH chief executive, said: "As a nation we’re failing the test on childhood obesity.
"Our projections show that we are heading in the wrong direction on obesity, with children in some of the most deprived areas set to be worst affected."
He added: "Making physical activity the default in schools will go a long away in helping to build a healthier future."
He said rather than physical activity happening for just two hours a week in PE, it should be an "integral part of the whole curriculum".
Ali Oliver, chief executive of the Youth Sport Trust, said supporting children's mental and social development is also needed to reverse the trend in childhood obesity.
She added: "We now need a new national plan which fully embraces the power of physical activity including PE, sport and play, to develop a generation of children and young people who are happy, healthy and ready to learn."
The Government said it is determined to give every child the "best start in life", which it added includes creating the "healthiest generation of children in our history".
A spokesperson said: "That is why we are acting quickly with experts across the sector to revise the School Food Standards, so every school is supported with the latest nutrition guidance.
"We are also considering how best to build physical activity into the lives of children and adults as part of our mission to boost the nation’s health."
They added: "More widely, we are urgently tackling the childhood obesity crisis by shifting our focus from treatment to prevention, including by limiting school children's access to fast food and cracking down on junk food advertising."