Dark chocolate can reduce the risk of a heart attack or stroke by up to 20 per cent, the study suggests.

But the effect is only seen in people who already have high blood pressure.

Researchers at the University of Adelaide found that dark chocolate “significantly” decreased blood pressure in subjects with hypertension.

The benefits stem from chemicals called flanavols, which are present in high quantities in cocoa beans.

The chemical stimulates the production of endothelial nitric oxide, which causes blood vessels to dilate.

The research concluded that eating food high in flanavols could have an effect on blood pressure equivalent to that produced by 30 minutes of exercise.

But the study stopped short of recommending the replacement of hypertension drugs with a chocolate diet.

The sugar and fat found in the snacks could have a harmful effect on the heart, they said.

“The practicability of chocolate or cocoa drinks as long-term treatment is questionable,” said researcher Dr Karin Ried.

“There have, however, been conflicting results as to the real-life effects of eating chocolate.

“We’ve found that consumption can significantly, albeit modestly, reduce blood pressure for people with high blood pressure but not for people with normal blood pressure.”

The research is published in the journal BMC Medicine.

Mike Rich, Executive Director of the Blood Pressure Association, said: “This is interesting research, which seems to indicate that eating dark chocolate can have a moderate blood pressure lowering-effect on those who already have high blood pressure.

“But more research is needed to see if this is anything more than a short-term effect. Of course, there’s nothing wrong with the occasional sweet treat, but eating too much chocolate may cause weight gain, which in turn raises blood pressure, so we wouldn’t recommend people rush out to get giant bars of dark chocolate just yet.”

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telegraph.co.uk