Most UK adults are so unwilling to exercise that not even the threat of an early death is enough to get them off the sofa, a survey suggests.
Lack of exercise increases the risk of heart disease and cancer
Only 38% of people questioned by YouGov said they would do more exercise if their life depended on it.
And British Heart Foundation figures show only a third of people manage to do enough exercise to achieve the minimum recommended amount.
Experts warned inactivity is dangerous even in those who are a healthy weight.
Among the 2,100 people surveyed, brisk walking was found to be the favourite way of getting exercise - before dancing, swimming or going to the gym.
However, only 4% said they found exercise fun.
A greater inspiration was exercising to change body shape, particularly among women and young adults.
Almost a third of 18 to 24-year-olds reported they would do more exercise if they saw an unflattering photo of themselves or were told they looked fat.
Other less predictable forms of motivation to work out included fancying someone at the gym.
But only 13% of men and 7% of women said keeping a healthy heart was their main motivator.
Excuses for not exercising were found to be always close at hand - from not having enough time to the one in seven who blame bad weather for not doing enough physical activity.
The British Heart Foundation, which is launching a campaign to encourage people to up their heart rate for 30 minutes a day, says that someone dies every 15 minutes as a direct result of physical inactivity.
Dr Mike Knapton, director of prevention and care at the BHF, said it was a "deadly serious" problem.
"With our busy lifestyles and labour-saving devices we've stopped getting the exercise our bodies desperately need.
"For many people, exercise has become an ugly word, something to avoid at all costs - but you'd be amazed how easy it is to up the tempo of your heartbeat.
"Just 30 minutes a day will do you and your heart the world of good."
The government recommends a minimum of 30 minutes of moderate-intensity physical activity five times a week.
Dr David Haslam, clinical director of the National Obesity Forum, said it made for depressing reading but confirmed what had been shown in clinical trials, where even those who had a heart attack did not change their lifestyles.
"Children instinctively exercise when left to their own devices, but they don't because they're stopped from doing that by the school curriculum and parents scared of child abductors and murderers lurking on every corner.
"So, if it doesn't become a habit, you're not going to work hard to go against the tide and introduce it as an adult."
He added that exercise could be incorporated into everyday life.
"Physical activity and obesity are two different risk factors, so even if you're lean, if you're inactive you increase your risk of cancer and cardiovascular disease," he said.