Parents who allow their children to eat junk food and lead sedentary lifestyles are "normalising obesity", the head of the NHS in England has warned.
Simon Stevens said youngsters could fall victim to a "rising tide" of health conditions such as diabetes, heart disease and cancer unless families, along with the health service and the Government, stepped up prevention.
In an international study published last week, more than a third (36%) of UK adults thought they were simply overweight when they were actually clinically obese, while research in th e British Journal of General Practice in March found just under a third (31%) of parents underestimated their child's weight.
The NHS England chief executive also said smoking was mostly behind a gulf in life expectancy between rich and poor.
In a speech in the West Midlands, he said: "First, as a nation it's time to get our act together on prevention.
"Yes, life expectancy is its highest ever. But smoking still explains half the inequality in life expectancy between rich and poor - and two thirds of smokers get hooked as kids. Binge drinking costs at least £5 billion a year - in A&E admissions, road accidents, extra policing.
"Junk food, sugary fizzy drinks and couch potato lifestyles are normalising obesity - and as parents, a third of us can't now spot when our own child is seriously overweight.
"So we've got a choice. Condemn our children to a rising tide of avoidable diabetes, cardiovascular disease, cancer? And burden taxpayers with an NHS bill far exceeding an extra £8 billion by 2020? Or take wide ranging action - as families, as the health service, as government, as industry. Using the full range of tools at our disposal.
"It's a no brainer - pull out all the stops on prevention, or face the music."
Speaking alongside David Cameron as the Prime Minister announced plans to extend NHS services for seven days a week yesterday, Mr Stevens added he felt the health service "was entering probably the most challenging period in its 67-year history".
The Conservatives made clear during the election campaign they supported a plan put together by Mr Stevens to fill a funding gap estimated at £30 billion a year by 2020.
The scheme would see £22 billion covered through efficiencies, with the £8 billion remainder coming from government funds.