LONDON (Reuters) - People working 10 or 11 hours a day are more likely to suffer serious heart problems, including heart attacks, than those clocking off after seven hours, researchers said on Tuesday.

The finding, from an 11-year study of 6,000 British civil servants, does not provide definitive proof that long hours cause coronary heart disease but it does show a clear link, which experts said may be due to stress.

In all, there were 369 cases of death due to heart disease, non-fatal heart attacks and angina among the London-based study group -- and the risk of having an adverse event was 60 percent higher for those who worked three to four hours overtime.

Working an extra one to two hours beyond a normal seven-hour day was not associated with increased risk.

“It seems there might a threshold, so it is not so bad if you work another hour or so more than usual,” said Dr Marianna Virtanen, an epidemiologist at the Finnish Institute of Occupational Health and University College London.

The higher incidence of heart problems among those working overtime was independent of a range of other risk factors including smoking, being overweight or having high cholesterol.

But Virtanen said it was possible the lifestyle of people working long hours deteriorated over time, for example as a result of poor diet or increased alcohol consumption.

More fundamentally, long hours may be associated with work-related stress, which interferes with metabolic processes, as well as “sickness presenteeism,” whereby employees continue working when they are ill.

Virtanen and colleagues published their findings in the European Heart Journal.

Commenting on the study, Gordon McInnes, professor of clinical pharmacology at the University of Glasgow’s Western Infirmary, said the findings could have widespread implications for doctors assessing patients’ heart risks.

“If the effect is truly causal, the importance is much greater than commonly recognized. Overtime-induced work stress might contribute to a substantial proportion of cardiovascular disease,” he said.