Researchers said the impact of such lifestyles on conditions such as heart disease, diabetes and cancer amounted to around £0.7bn of NHS spending annually.
Previous studies have suggested that spending a day sitting at a desk, without any extra physical activity, could increase the risk of premature mortality by up to 60 per cent.
Lead Investigator, Leonie Heron from the Centre of Public Health at Queen’s University Belfast, said: “Many individuals in the UK spend their leisure time in sedentary behaviour, and the workplace represents a significant proportion of unavoidable daily sitting time for many people.”
Steven Ward, chief executive of ukactive, said people should get moving, warning “sitting still kills.”
“It doesn’t have to mean major lifestyle changes – building activity into everyday life is a great place to start, such as taking the opportunity to stand on the bus rather than take a seat, or answering phone calls at work standing up rather than sitting down,” he said.
Prof Naveed Sattar, Professor of Metabolic Medicine, University of Glasgow, said those whose jobs meant a lot of sitting down should make an effort to be active at other times of the day, or try to take regular short walks.
He said: “People should aim to accumulate more activity commuting to their works or during their evenings. Even adding an extra 1000 steps per day is a reachable target for most if they feel they are not getting enough activity.
Dr Gavin Sandercock, Reader in Clinical Physiology (Cardiology) and Director of Research, University of Essex, said: “We know sitting too much can be bad for health but this is the first really detailed look at the cost of problem in the UK.
The move from manufacturing to a predominantly service-based industry means that people sitting at desks are now the backbone of the UK economy.
Sitting less might save some lives and cost the NHS less but, because we have created a sitting-based economy, there are likely to be costs associated with interventions to reduce sitting-time in the workplace,” he said.