Last updated at 20:58 08 November 2007

Sitting down for extended periods poses a health risk as "insidious" as smoking or over-exposure to the sun, a scientist claims.

Millions of workers who sit at desks for hours on end before going home to sit in front of the TV are increasing their chances of heart disease, Type 2 diabetes and obesity, says Professor Marc Hamilton of the University of Missouri.

Writing in the November issue of the medical journal Diabetes, he said: "The dire concern for the future may rest with growing numbers unaware of the potential insidious dangers of sitting too much.

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"These studies demonstrate a significant impact of inactivity on a par with smoking.

"I view exposure to sitting...like an oncologist views exposure to unnecessary sunlight."

A second study at the International Diabetes Institute in Melbourne concluded that even two hours of exercise a day would not compensate for "spending 22 hours sitting on your rear end".

Professor David Dunstan told Australian Doctor magazine: "People who break up their sitting time - walking to a colleague rather than emailing him, for instance - have a lower risk of diabetes."