The likely toll of long working hours is revealed in a major new study which shows that employees still at their desks into the evening run an increased risk of stroke – and the longer the hours they put in, the higher the risk.

The largest study conducted on the issue, carried out in three continents and led by scientists at University College London, found that those who work more than 55 hours a week have a 33% increased risk of stroke compared with those who work a 35- to 40-hour week. They also have a 13% increased risk of coronary heart disease.

The findings will confirm the assumptions of many that a long-hours culture, in which people work from early in the morning until well into the evening, with work also intruding into weekends, is potentially harmful to health.

The researchers, publishing their findings in the Lancet medical journal, say they cannot state categorically that long hours cause people to have strokes – but their study shows that there is a link, and it gets stronger as thehours people put in get longer.

“Sudden death from overwork is often caused by stroke and is believed to result from a repetitive triggering of the stress response,” they write. “Behavioural mechanisms, such as physical inactivity, might also link long working hours and stroke; a hypothesis supported by evidence of an increased risk of incident stroke in individuals who sit for long periods at work.

“Physical inactivity can increase the risk of stroke through various biological mechanisms and heavy alcohol consumption – a risk factor for all types of stroke – might be a contributing factor because employees working long hours seem to be slightly more prone to risky drinking than are those who work standard hours.”

People who work long hours are also more likely to ignore the warning signs, they say – leading to delays in getting treatment.

Mika Kivimäki, professor of epidemiology at UCL, and colleagues looked separately at heart disease and at stroke. For coronary heart disease, they pulled together 25 studies involving more than 600,000 men and women from Europe, the USA and Australia who were followed for an average of 8.5 years.

They then pooled and analysed the data that had been collected. This produced the finding of a 13% increase in the chances of a new diagnosis of heart disease or hospitalisation or death.

For stroke, they analysed data from 17 studies involving nearly 530,000 men and women who were followed up for an average of 7.2 years. They found a 1.3 times higher risk of stroke in individuals working 55 hours or more, compared with those working a standard 35- to 40-hour week.

This association remained even after taking into account health behaviours such as smoking, alcohol consumption and physical activity as well as standard cardiovascular risk factors including high blood pressure and high cholesterol.

The longer the working week, the higher was the risk of stroke. Those working between 41 and 48 hours had a 10% higher risk of stroke and those working 49 to 54 hours had a 27% increased risk.

Kivimäki said the scale of the study allowed the team to be more precise about the health toll of long hours than ever before. He suggested that doctors should take note of the possible risks to their hard-working patients. “Health professionals should be aware that working long hours is associated with a significantly increased risk of stroke, and perhaps also coronary heart disease,” he said.

In a commentary in the journal, Dr Urban Janlert from Umeå University in Sweden writes that the European Working Time Directive, meant to limit the week to 48 hours, is not in effect in all countries. “Long working hours are not a negligible occurrence. Among member countries of the OECD, Turkey has the highest proportion of individuals working more than 50 hours per week (43%) and the Netherlands the lowest (less than 1%).

“Although some countries have legislation for working hours ... it is not always implemented. Therefore, that the length of a working day is an important determinant mainly for strokes, but perhaps also for coronary heart disease, is an important finding.”

Dr Tim Chico, reader in cardiovascular medicine at the University of Sheffield, said the study did not prove long working hours could cause stroke or heart disease. “It is almost certainly impossible to prove whether there is a direct link as this would require thousands of people to be randomly allocated to work more or less hours and followed up for years to see if this changes the risk of stroke, while keeping all other behaviours the same between groups,” he said.

For many people, cutting down on working hours would be difficult or impossible, he said. “Most of us could reduce the amount of time we spend sitting down, increase our physical activity and improve our diet while working and this might be more important the more time we spend at work. We should all consider how the working environment could be altered to promote healthy behaviour that will reduce strokes, irrespective of how long we work.”

• This article was amended on 24 August 2015 because the European Working Time Directive is meant to limit the week to 48 hours, not the day to 48 hours as an earlier version said.