Being overweight is so common in Europe that it risks becoming "the new norm", with around a third of teenagers now heavier than is recommended for their health.
"Our perception of what is normal has shifted; being overweight is now more common than unusual. We must not let another generation grow up with obesity as the new norm," said Zsuzsanna Jakab, the WHO's ( World Health Authority) regional director.
She blamed a combination of high levels of physical inactivity, coupled with a culture that promotes cheap, convenient foods high in sugars, fats and salt. This combination, she said, "is deadly".
Lack of exercise is a key part of the problem.
The WHO recommends children aged 5 to 17 should get at least 60 minutes of moderate to vigorous physical activity a day, and adults should do at least 150 minutes of moderate exercise a week.
Joao Breda, a WHO expert on nutrition, physical activity and obesity, said peoples' living environments - including the layout of town, cities, schools and workplaces - are crucial to increasing rates of exercise.
"We need to create environments where physical activity is encouraged and the healthy food choice is the default choice, regardless of social group," he said in a statement released with the report.
"Physical activity and healthy food choices should be taken very seriously in all environments - schools, hospitals, cities, towns and workplaces. As well as the food industry, the urban planning sector can make a difference," he added.
The WHO, however, that some countries, including France and some Scandinavian countries, have managed to contain the obesity epidemic "through a whole-of-government approach".
It said many policies in these countries - such as promoting vegetable and fruit consumption in schools, taxing certain foods to reduce intake, controlling advertising, employing good surveillance and monitoring, and taking action to promote physical activity - had combined to help keep obesity levels stable.