Inactivity is killing nearly 70,000 people in the UK every year, a study has found.
Experts warn Britain runs on a 'sitting-based economy' in which the vast majority of working adults spend their days bound to a desk.
Some 30 per cent of us spend at least six hours a day seated during the week.
But tellingly, at the weekend this figure goes up to 37 per cent, suggesting we are even lazier in our leisure time.
Researchers at Queen's University Belfast warned that so much sedentary behaviour is causing thousands of cases of heart disease, type two diabetes, cancer and even death.
The academics, writing in the BMJ Journal of Epidemiology and Community Health, calculated the NHS is spending £762million a year dealing with the consequences of people lounging around.
Studies have shown that spending large parts of the day sitting down increases the risk of cardiovascular disease, type 2 diabetes, cancer and death and is a burden on health services
They said people who spend the much of their day planted in one place have an 88 per cent increased risk of type two diabetes, a 14 per cent increased risk of heart disease and a 25 per cent increased risk of an early death.
The risks of lung cancer go up by 27 per cent, bowel cancer by 30 per cent and womb cancer by 28 per cent.
They calculated that one in nine of all deaths seen in the UK every year - 69,276 in 2016/17 alone - are linked to sedentary behaviour.
Study leader Leonie Heron from the Centre of Public Health at Queen's University Belfast, said: 'We know that spending large parts of the day sitting down increases the risk of a number of illnesses including cardiovascular disease and diabetes.
'This research has enabled us to put a figure on this, demonstrating the huge cost this has on the NHS and highlights the pressing need to address this issue to both reduce the financial cost and improve population health.
'Our research showed that sedentary behaviour contributed to almost 70,000 lives lost in 2016.
SITTING FOR TOO LONG IS NOT AS BAD AS SMOKING
Sitting for too long is not as bad as smoking, researchers declared in November in hope of debunking the widespread myth.
Evidence has already shown inhaling tobacco smoke boosts the risk of premature death from any cause by around 180 per cent.
However, studies show the heightened risk of an early grave from excessive sitting - more than eight hours a day - is 20 per cent, at most.
Epidemiologists at the University of South Australia, led by Dr Terry Boyle, warned the two are simply not comparable.
'This could have been avoided if sedentary behaviour was eliminated in the UK.'
Her team calculated too much sitting was linked to 17 per cent of all cases of type two diabetes, five per cent of heart disease, seven per cent of lung cancer, nine per cent of bowel cancer and eight per cent of womb cancer.
Health officials are particularly worried about the millions of inactive middle-aged people who continuously put their busy lives ahead of the needs of their health.
They have repeatedly warned that Britain is the midst of an 'inactivity epidemic' - with poor health awaiting many of those who do no exercise at all.
Dr Mike Brannan, national lead for physical activity at Public Health England said: 'Even if you are physically active, sitting for long periods of time damages your health and greatly increase your risks of a broad range of health conditions.
'People should sit less and move more. But there are many situations in life - such as workplaces and care settings - where sitting is inevitable, so it is important to take regular breaks by standing or moving around.'
Experts are divided on exactly what it is about prolonged sitting that is so harmful.
Some think it is simply a lack of exercise that does the damage.
But others believe the very act of being still and not moving your muscles for hours at a time can be dangerous in the long term - even if you go to the gym every evening.
Dr Gavin Sandercock, director of research at the University of Essex, said Britain now runs on a 'a sitting-based economy'.
'The move from manufacturing to a predominantly service-based industry means that people sitting at desks are now the backbone of the UK economy.'
But he added: 'You actually have to reduce sitting time by several hours each day to see noticeable improvements in health.
'In contrast - getting people to be more physically active has much bigger effects.'
Professor Naveed Sattar of the University of Glasgow, said: 'People should not worry about sitting down at work if that's what they have to do.
'The better message to communicate is people should aim to accumulate more activity commuting to their works or during their evenings.
'Even adding an extra 1,000 steps per day is a reachable target for most if they feel they are not getting enough activity.'