Just being overweight, like two thirds of UK adults, was found to raise the risk of a whole week off sick by a fifth.
The risk rose to more than double among the heaviest citizens.
The findings, from a study of 129,000 people in 29 countries, were presented at the European Congress on Obesity.
Researchers say Britain, where 60 per cent are overweight or obese, is likely to be worst hit.
Study author Dr Thomas Czypionka, from the Institute for Advanced Studies in Vienna, said: “There are so many health consequences that come from being obese, such as high blood pressure, diabetes, arthritis and sleep apnoea.
“Even being overweight increases the risk.
“In the UK over 60 per cent are overweight or obese and all of these people have increased health risks and some of them have diseases that reduce their ability to work.
“I don’t think it is a good idea to just wait and let people get obese and then pay for the treatment, we need measures to reduce obesity.”
Co-author Siegfried Eisenberg added: “The UK is a region where you clearly can see the impact of obesity on absenteeism because the prevalence is higher compared to other countries.
“The impact on the economy increases with the prevalence of obesity so it is a serious problem for the UK.”
Ministers plan to overhaul the sick note system to curb the number being signed off by their GPs.