The study by British government think-tank Foresight called for greater help to counter the "'obesogenic' environment" by designing towns and cities to promote walking and cycling and encouraging people to buy healthier food.
But it could take 30 years to tackle the problem, it said. Obesity rates have more than doubled in Britain in the last 25 years -- in 2004, nearly a quarter of men and women in England were obese.
"There is compelling evidence that humans are predisposed to put on weight by their biology," the report said.
"Although personal responsibility plays a crucial part in weight gain, human biology is being overwhelmed by the effects of today's 'obesogenic' environment, with its abundance of energy-dense food, motorised transport and sedentary lifestyles.
"As a result, the people of the UK are inexorably becoming heavier simply by living in the Britain of today."
Some experts said the report confirmed what the government had known for years and accused it of failing to act.
Peter Hollins, chief executive of the British Heart Foundation (BHF), said it was "hardly a wake-up call".
"Reports like this, which should have had alarm bells ringing... long ago, have been met only by repeated pushes of the government's snooze button," he said.
Britain on Monday launched a campaign for greater participation in sports at school to combat the looming obesity crisis, which Health Secretary Alan Johnson was was potentially on the scale of climate change.
Government-commissioned research suggested half of all Britons will be obese in 25 years if current trends are not halted; furthermore, 86 percent of men will be overweight in 15 years and 70 percent of women in 20, it suggested.