Researchers said a long-term study showed that working more than 11 hours a day increased the risk of heart disease by 67 percent, compared with working a standard 7 to 8 hours a day.

They said the findings suggest that information on working hours -- used alongside other factors like blood pressure, diabetes and smoking habits -- could help doctors work out a patient's risk of heart disease.

However, they also said it was not yet clear whether long working hours themselves contribute to heart disease risk, or whether they act as a "marker" of other factors that can harm heart health -- like unhealthy eating habits, a lack of exercise or depression.

"This study might make us think twice about the old adage 'hard work won't kill you'," said Stephen Holgate, chair of the population and systems medicine board at Britain's Medical Research Council, which part-funded the study.

The study, published in the Annals of Internal Medicine journal, followed nearly 7,100 British workers for 11 years. 

Cardiovascular diseases such as heart attacks and strokes are the world's largest killers, claiming around 17.1 million lives a year, according to the World Health Organization (WHO). Billions of dollars are spent every year on medical devices and drugs to treat them.