Everyone knows that being in an unhappy marriage can make people miserable.

But now, researchers have suggests it could even prove fatal.

US researchers believe that an unhappy marriage can cause stress which, in turn, affects physical health. They believe people who are unhappy with their spouse could be at higher risk of depression, high blood pressure and even heart disease.

As a result, the scientists at Michigan State University are set to study how marriage affects cardiovascular health.

They say this is particularly important at a time when one in four people in the US die of heart disease.

Dr Hui Liu, the lead researcher, said: "The importance of this study is highlighted by the continued high prevalence of cardiovascular disease in the US. We plan to provide representative evidence on how marriage affects cardiovascular health and elucidate the multiple mechanisms in this relationship.”

The researchers say most cases of heart disease are preventable meaning identifying risk factors is crucial to the design of effective prevention strategies.

Dr Liu said scientists have long believed marriage to be the most important social relationship affecting health.

She believes, for example, that a happy marriage offers support and enhances physical health. In contrast, she says, the stress of an unhappy marriage can cause depression.

This, she says, can promote unhealthy habits, such as smoking and drinking, and can increase the body’s levels of stress hormones.

These hormones can result in raised blood pressure and heart rate eventually leading to heart disease.