Research on more than one million adults found that sitting for at least eight hours a day could increase the risk of premature death by up to 60 per cent, reports The Telegraph.
Scientists said sedentary lifestyles were now posing as great a threat to public health as smoking, and were causing more deaths than obesity.
They urged anyone spending hours at their desk to change their daily routine to take a five minute break every hour, as well as exercise at lunchtimes and evenings.
An hour of brisk walking or cycling spread over a day was enough to combat the dangers of eight hours sitting in the office, they said.
Currently, public health advice in the UK recommends just half this level of activity.
But almost half of women and one third of men fail to achieve even this.
Lead scientist Professor Ulf Ekelund, from Cambridge University and the Norwegian School of Sports Sciences, said: “We found that at least one hour of physical activity per day, for example brisk walking or bike cycling, eliminates the association between sitting time and death.”
A decent walk – at a speed of just over three miles an hour – was enough to achieve the benefit, he stressed.
“You don’t need to do sport, you don’t need to go to the gym, it’s OK doing some brisk walking maybe in the morning, during your lunchtime, after dinner in the evening. You can split it up over the day but you need to do at least one hour,” he said.
Researchers said the typical modern lifestyle of spending a day in front of a computer, followed by an evening slumped in front of the television was proving fatal.
They also called for radical changes in Government policies, to encourage healthier habits.
These include placing bus stops further apart to force people to walk longer to and from them,
closing streets to cars at weekends to encourage more sports and exercise and opening free public gyms in parks.
Researchers said many office workers, especially commuters, would find it hard to avoid long periods of being seated, but should make every effort to break up their day, with short walks.
“Take a five minute break every hour, go to the next office, go upstairs to the coffee machine, go to the printer,” said Prof Ekelund. “Build physical activity in your everyday life.”
Fellow researcher Dr Pedro Hallal, from the Federal University of Pelotas in Brazil, said: “The combination of sitting too much all day, and too little activity is deadly.”
If office workers wanted to offset the damage caused by a day at their desks, they needed to do double the amount of exercise recommended by health officials in the UK, he said.
Researchers urged employers to make it easier for workers to take more exercise, providing showers and gyms, and encouraging longer breaks.
The studies could not pinpoint why long periods of sitting were specifically risky.
But the scientists involved said movement appeared to assist the body’s metabolism, while sedentary periods could influence hormones such as leptin, which regulate energy balance.
In the study, participants, mostly aged over 45, were classed by their levels of physical activity – from up to five minutes a day to more than an hour – and by the amount of time spent seated.
This was compared with death rates over up to 18 years among the adults, who came from western Europe, Australia and the US.
Among those who sat for at least eight hours daily and managed less than five minutes’ activity mortality rates were 9.9 per cent.
But those who spent just as long seated, but managed at least an hour’s exercise, saw death rates drop to 6.2 per cent.
Cancer and heart disease were the two most likely causes of death linked to inactivity.
Researchers said that around one hour’s activity a day was enough to reverse the damage caused by prolonged sitting.
Similar results were found when the scientists looked at the television viewing habits of a subgroup of about 500,000 people. Watching TV for more than three hours per day was associated with an increased risk of death in all groups except those who managed at least an hour’s exercise.
The research, from 16 studies, is among four papers published in The Lancet today ahead of the Olympic Games in Brazil.
One found that physical inactivity is costing the UK £1.7bn a year, while another showed that the London 2012 Olympics had little impact on public fitness levels.
Researchers said that globally, more than 5 million deaths a year are linked to physical inactivity – a similar number to lives lost to smoking, and a higher figure than that caused by obesity.
Steven Ward, executive editor of UK Active, urged employers to do more to encourage workers to be more active during the working day.
He also urged workers to do all they could to find time to get moving.
“This report is showing that inactivity kills,” he said. “When we realised this about smoking we tackled it – we need to do the same about our office culture.”
He called for changes in tax breaks, to encourage office workers to be more active, by providing free gym membership, or activity trackers, in the same way that Cycle to Work schemes let employers loan out bicycles as a tax-free benefit.
Polls of office workers have found that just one in five leave their workplace at lunchtime, while just three per cent use the time to visit the gym.