Scientists warn that it can trigger a vein condition known as venous thromboembolism.
It usually affects the legs, arms or pelvis but can lead to clots in the lungs.
And US researchers warned that exercise does not slash the risk – unless it is regular.
Professor Mary Cushman, of Vermont University, in the US, said: “Watching TV itself isn’t likely to be bad but we tend to snack and sit still for prolonged periods while watching.”
The researchers asked 15,158 participants aged 45 to 64 about how often they viewed TV.
Rates of the condition were 70 per cent higher among those answering “very often”, compared with those who said “never or seldom”.
Prof Cushman said: “Physical fitness is protective. The association of TV viewing with thrombosis was independent of this, though.
Think about how you can make the best use of your time to live a fuller and healthier life.
You could put a treadmill or stationary bike in front of your TV and move while watching. Or you can delay watching TV by 30 minutes while you take a walk.
“If you must see your favourite show, tape it while you are out walking so you can watch it later, skipping the ads.”
If the clots develop in the lungs, or travel there from other areas such as the legs, they can turn into a blockage known as an pulmonary embolism.
This can cause death by cutting off oxygen and blood supply.
Prof Cushman said although TV viewing promotes obesity, which can cause clots, there are also issues with prolonged sitting impairing circulation in the legs, “setting the stage” for blood pooling in veins.
“If you are at heightened risk of venous thromboembolism due to a recent operation, pregnancy or recent delivery, cancer or a previous clot, your doctor may prescribe blood-thinning medication or advise you to wear compression stockings,” she said.
The condition begins in a vein and includes two types of blood clots – deep vein thrombosis which forms in the leg and pulmonary embolism.
US heart and vascular expert Professor Mark Creager said: “The reason sitting and watching TV would likely put someone at risk is when they don’t move their legs.
“When we walk our calf muscles actually squeeze the veins, almost milk them, to keep the blood moving along.”
Each year about 25,000 people in Britain die from the condition.