Sitting for more than ten hours a day gives women a 'biological age' up to eight years older than it should be, according to a major study.
The researchers, who tracked the movements of 1,481 women over the age of 64, found a strong link between a sedentary lifestyle and the premature ageing of cells in the body.
This process is known to increase the risk of cancer, diabetes and heart disease.
Just half an hour of moderate exercise - such as brisk walking, gardening or cycling - is enough to undo the damage of a day sat down.
Experts last night said the findings should come as a wake-up call to people who spend hours on end without moving.
People in Britain are known to spend an average of nine waking hours sitting a day.
Those who work in sedentary jobs are likely to spend even more of their day without moving, with those at particular risk including aeroplane pilots, taxi drivers and office workers, who spend an average of 75 per cent of their working day sitting in front of a computer screen.
Dr Aladdin Shadyab, of the University of California, San Diego, led the new study, said: 'Our study found cells age faster with a sedentary lifestyle.
'Chronological age doesn't always match biological age.'
Dame Sally Davies, the Government's chief medical officer, has called inactivity 'a silent killer'
She advises adults spend 150 minutes a week – two and a half hours – in moderate activity in bursts of ten minutes or more.
For the new study the San Diego team tracked women for a week using accelerometers - small gadgets attached to the belt which recorded every movement.
Discussions about the benefits of exercise should start when we are young, and physical activity should continue to be part of our daily lives as we get older, even at 80 years old.'
The research team analysed elderly women because they were already taking part in a long-running US study called the Women's Health Initiative.
They are now planning further studies to see whether the same findings would apply to men.