A study has found that a regular 2oz dose can lower blood pressure and reduce the risk of having a heart attack or stroke - without any weight gain.

But before you reach for the king-size bar, beware. The effects are cancelled out if more than 30 calories are consumed each day, the researchers warn.

It was already known that chemicals in chocolate called polyphenols can reduce blood pressure.

But the German study shows that the fat content does not cancel out the beneficial effects - at least where small portions are concerned.

Blood pressure lowered in this way could reduce the risk of death from a stroke by 8 per cent and from coronary artery disease by 5 per cent, the University of Cologne scientists found. It would also reduce risk of death generally by 4 per cent.

A daily diet of a little chocolate had the same effect as more complicated diets designed to lower blood pressure, according to the authors of the study, published in the Journal of the American Medical Association.

"Although the magnitude of the blood pressure reduction was small, the effects are clinically noteworthy," they said. "Adoption of small amounts of flavanol-rich cocoa into the habitual diet is a dietary modification that is easy to adhere to, and therefore may be a promising behavioural approach to lower blood pressure in individuals with above-optimal blood pressure."

However, Sara Stanner, a nutritionist at the British Heart Foundation, was cautious.

"This study suggests that small quantities of dark chocolate might have beneficial effects on blood pressure. However, it is important to remember that chocolate is also high in fat and calories - so over-indulgence is not good for your heart."

Polyphenols are also found in fruit and vegetables - and can be consumed without the risks associated with over-indulging, she added.