Sitting at a desk all day, driving to work and lazing on the sofa at weekends can trigger a decline towards ill health in just two weeks, a study found.
Health begins to deteriorate within a fortnight of living a "couch potato" lifestyle, according to University of Liverpool researchers.
They warned that Britons' increasingly inactive lives could damage their health in the long term, potentially leading to serious conditions such as type 2 diabetes, heart disease and strokes, reports Daily Mail.
But the good news is the ill-effects can easily be reversed with simple steps like taking the stairs instead of the lift, getting off the bus a stop early and going to the supermarket instead of shopping online.
Lead researcher Dr Daniel Cuthbertson warned a sedentary lifestyle quickly begins to "sow the seeds for future disease".
He said: "Through various advances, our society has become a lot more sedentary.
"Our grandparents did washing manually, they did manual labour, people were physically active. Now many of us are based at desks tapping away – we don't even go out to do our shopping.
"Sedentary behaviour has a number of adverse health effects that over an extended period of time could be harmful."
For the study, researchers followed 45 people with an average age of 36 who did not regularly do any active exercise like jogging or going to the gym but all walked at least 10,000 steps a day as part of their daily lives.
They were asked to become inactive for two weeks. This involved getting public transport or driving to work, taking lifts or escalators instead of stairs, cutting their steps down to around 1,500 a day and spending the weekend at home, mainly watching TV or playing computer games.
Researchers checked participants' activity levels using a tracker on their arms. All were also asked to stick to their normal diets and keep a food diary to show they had not changed what they ate.
After two weeks, tests showed participants had increases in their fat levels and waist sizes, and showed signs of muscle loss and lower cardio-respiratory fitness.
Their bodies were also less able to respond to the hormone insulin – a symptom which can be a precursor to developing diabetes.
After participants had resumed their normal activity levels for 14 days, the negative effects were reversed.
Dr Cuthbertson said: 'Even in two weeks, the transition from being a busy bee to a couch potato provoked subtle changes which over months or years would predispose people to certain diseases.
"While these small changes would not cause disease in their own right in two weeks, imagine this over a longer period of time where there may be a cumulative effect and perhaps an added effect of a poor diet."
Dr Kelly Bowden Davies, who conducted the study, said: "We saw this effect in young, healthy individuals who were normally active so we need to think about what happens to people who are sedentary all the time or who are 20 years older."
She added: "People really underestimate how simple things like increasing their daily steps or changing the amount of time they spend sitting around can change their health.
"We're always told to exercise and go to the gym but we've shown that these subtle changes also have quite an effect."
The results also suggested being physically active could be even more important for people who had an immediate relative with type 2 diabetes.
The research, published in the journal Diabetologia, was funded by Diabetes UK.
Dr Elizabeth Robertson, director of research at the charity, said: "This study sheds light on the potential harmful effects that short-term inactivity could have on our health, especially if we have a close family history of Type 2 diabetes.
"Living an active lifestyle and eating a healthy balanced diet are the key ways to reduce your risk of Type 2 diabetes."
Official NHS guidance recommends adults exercise for 30 minutes, five days a week but many people fail to meet this target. Figures from Sport England suggest one in four people fail to do even 30 minutes of activity a week.
WHAT ARE THE FIVE STEPS TO STAYING ACTIVE?
1. Take the stairs instead of the lift or escalator wherever possible.
2. Walk to work, get off the bus a stop early or park away from the office so you have to walk in.
3. Move the printer or water cooler away from your desk so you have to get up regularly during the working day.
4. Do your weekly shop at the supermarket instead of shopping online.
5. Don't spend the whole weekend on the sofa - mix up relaxing with activities like cleaning the house, shopping or going for a walk.