The researchers, who found four in ten children aged five to 19 are overweight, have warned of an "absolute crisis" in child heath including a greater future risk of heart disease, cancer and diabetes.
"The trend predicts a generation of children and adolescents growing up obese," says Majid Ezzati, the Imperial College professor who led the study and who has called for better regulations and taxes to protect children from junk food.
Dr Fiona Bull from the World Health Organisation blames politicians for failing to act after years of warnings and said: "Obesity is a global health crisis today, and threatens to worsen in coming years unless we start taking action. We are surrounded by environments that market unhealthy, high-fat, high-sugar, high-calorie food. That's what is on the TVs, that's what is promoted at bus stops."
Some are taking notice, and just this week Pizza Hut and TGI Fridays announced plans to stop selling refillable sugary drinks by next March in a bid to reduce children's sugar intake.
WHAT YOU CAN DO AS A PARENT
But do parents have a responsibility to act, too? Yes, according to Matt Roberts, personal trainer and father of two, who has worked with David and Samantha Cameron. "Things have gone too far, and today's parents are fearful of speaking to their child about their weight. But you can do it delicately, appropriately and rather than singling a child out you can tackle it as a household."
Matt says this means leading by example, and being honest about your own eating and exercise habits. "Have a kitchen detox so there aren't biscuits and soft drinks around to tempt them. Don't ban treats entirely, but limit to them to outside the house, where you have less control anyway.
"Secondly, get off the sofa yourself. Both sign up for something like junior park run (parkrun.org.uk). Cycle or dog walk together and make it social by inviting their friends along, too. Encourage them into sports and try different ones until they find something they love."
Matt says it's no coincidence childhood obesity rates were lower in 1975. "There were no gadgets or social media to keep them inside and inactive. If your child is under eleven set screen time limits. But if you have a 14-year-old? Good luck with that. I have children myself, so I know it's hard. All we can do as parents is keep trying to engage with them, encourage them into sport and limit junk food at home."
Matt also advises talking in terms of health and not weight. "Don't tell them they need to lose a specific amount of weight – don't bring numbers into it. Rather, talk about how reducing sugar and getting out in the fresh air will make them feel energised and healthy, how it will make them better at their sport, and so on."
This approach helped Sally when her son Jack, now 18, became overweight in his early teens. "Unlike his younger brother, who is ridiculously skinny, Jack's always had a bigger build – plus he's greedy," she says. "When he was 13, he carried his weight really badly and looked very chubby, but he's sensitive and takes things to heart.
"I knew he was self-conscious about his trousers splitting and having to wear clothes two years older than his age. I sympathised because I was a chubby child and I remember how much it hurt when my mum tried to curb my own biscuit consumption. If I'd have said 'Stop being a pig' when he binged on biscuits, like my husband wanted to, it wouldn't have helped.
"One day, we were watching one of those health makeover shows and there was an overweight teenage boy on there who'd lost three stone. I started to cry and Jack said: 'You're worried that's going to be me, aren't you?' That was my opening, and we talked about the health dangers his weight could pose.
"Right away, he cut back on junk food and joined his local football team. He also began to grow into his shape out. Now he's over 6ft with broad shoulders."
WHY WE AVOID THE CONVERSATION
"I often find anxiety about talking to our children about weight are rooted in our own childhoods," says clinical psychologist Dr Rachel Andrew, author of The Supermum Myth. "I've rarely met a child who is overweight in a family where everybody is a normal weight. Often, one or more parents are overweight or have struggled with their weight in the past.
"These parents often worry about hurting their child's feelings or triggering an eating disorder, but I believe those parents will naturally be more sensitive to their child's feelings so they shouldn't worry."
Dr Andrew says parents should also remember some children may be more prone to weight gain than others and will therefore need an extra (but discreet) eye on them: "Lots of factors come into a child's weight, other than diet and exercise, like different body shapes, metabolisms and temperaments. Some children respond well to limits. Some children love playing football, whereas others like sedentary things like art."
And then, of course, there's the inextricable link between food and love. "As parents, we feel our primary responsibility is to nourish our child. But remember, not gently and kindly tackling a weight issue with them will do them more harm than good."
