LONDON (Reuters) - People who work 10 to 11 hours per day are more likely to suffer serious heart problems, including heart attacks than those who meet seven hours of work, researchers said on Tuesday.

The result of a 11-year study of 6,000 British civil servants do not provide definitive evidence that long working hours cause coronary heart disease, but it does show a clear association, which experts say could be due to stress.

In total, there were 369 cases of death from heart disease, nonfatal heart attacks and angina among the study group, based in London. And the risk of an adverse event was 60 percent higher among those who worked three to four hours extra.

Work an hour or two beyond the normal time of seven hours was not associated with an increased risk.

"It seems that there is a threshold, so not too bad if you work one or hours or a little longer than usual," said Dr. Marianna Virtanen, an epidemiologist at the Finnish Institute of Occupational Health and University College London.

The highest incidence of heart problems among workers who did overtime was independent of other risk factors as smoking, overweight or high cholesterol.

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

Above all, the long hours of work might be associated with increased job stress, which interferes with metabolic processes, like the "present with disease," in which employees will work even if they are sick.

Virtanen and his team 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 broad implications for doctors who evaluate their patients' heart risks.

"If the effect is truly causal, the importance is much greater than previously thought. Work stress induced by overwork can contribute to a substantial proportion of cardiovascular disease," he said.