Exclusion zones should be set up around schools to force parents and children to walk to class, a report suggests.
Obesity is rising because people walk less, the report says
The Institute for European Environmental Policy blames over-use of cars for fuelling the "twin crises" of global warming and an obesity epidemic.
Researchers found the amount of walking fell from two hours a week to just one when households started using a car.
They say banning cars around schools would instil good habits early on and call for "concerted action".
The IEEP study says that if people walked one more hour a week - the typical difference between those who own a car and those who do not - it could mean a reduction in average weight of up to two stone in a decade.
And the additional walking could displace at least 11 million tonnes of carbon dioxide from cars.
Other key findings from the report include:
- 40% of all journeys in the UK are under two miles
- 38% of these journeys are currently made by car
- Two miles is easily covered by 30 minutes of brisk walking
Lead author Dr Adrian Davis said: "The substitution of car use for walking is a major contributor to the steep rise in obesity, as walking is the most obvious way for most people to burn calories.
"A small daily reduction in walking over a decade or more has a profound and damaging impact on body weight."
Co-author Carolina Valsecchi said: "The twin crises of obesity and climate change are clearly interlinked through the switch from muscle power to engine power for transport. Concerted action is needed to reverse both these trends."
In addition to the exclusion zones for schools, the report also called for the government to invest heavily in making Britain's streets more attractive to walk in.
The report also quoted research from the US, which found that each additional hour spent in a car per day increased the likelihood of obesity by 6%.
The same study showed each additional kilometre walked per day reduced the chance of becoming overweight by 4.8%.
Another study in China showed that adults who travelled to work by car or motorbike doubled their likelihood of becoming overweight.
According to the report, the numbers of households without a car has dropped from 41% in the mid-1970s to 19%.
The number of hours the average Briton walks has also dropped from 87 hours a year to 67.