People working long hours are more likely to have a stroke, according to analysis of more than half a million people.

The data, published in the Lancet medical journal, showed the chance of a stroke increased beyond the traditional 9am to 5pm.

The link is uncertain, but theories include a stressful job and the damaging impact on lifestyle.

Experts said people working long hours should monitor their blood pressure

The study showed that in comparison to a 35-40 hour week, doing up to 48 hours increased the risk by 10%, up to 54 hours by 27% and over 55 hours by 33%.

Dr Mika Kivimaki, from University College London, said that in the 35-40 hour group there were fewer than five strokes per 1,000 employees per decade.

And that increased to six strokes per 1,000 employees per decade in those working 55 hours or more.

Dr Kivimaki admitted researchers were still at the "early stages" of understanding what was going on.

Ideas include the extra stress of working long hours or that sitting down for long periods is bad for health and may increase the risk of a stroke.

However, it could just be a marker for poor health with those chained to the office not having enough time to prepare healthy meals or exercise.

Dr Kivimaki told the BBC News website: "People need to be extra careful that they still maintain a healthy lifestyle and ensure their blood pressure does not increase."

The Stroke Association's Dr Shamim Quadir commented: "Working long hours can involve sitting for long periods of time, experiencing stress and leads to less time available to look after yourself.

"We advise that you have regular blood pressure checks, if you're at all concerned about your stroke risk you should make an appointment with your GP or health professional."

Dr Tim Chico, a consultant cardiologist based at the University of Sheffield, 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."