A few squares of dark chocolate every day might cut the risk of serious heart disease by helping to stave off the hardening of arteries, according to a study published on Tuesday.

Researchers from University Hospital in Zurich studied 20 male smokers, who are at greater risk of hardening arteries characteristic of coronary heart disease, to see the effects of dark and white chocolate on arterial blood flow.

The subjects, who were asked to abstain from eating foods rich in antioxidants for 24 hours, were given 40 grams (2 ounces) of chocolate to eat.

After two hours, ultrasound scans revealed that dark chocolate - made up of 74 percent cocoa solids - significantly improved the smoothness of arterial flow, whilst white chocolate, with four percent cocoa, had no effect, the study published in Heart magazine said.

The researchers, who said further studies were needed, suggested that the possible benefits arose from the antioxidants in dark chocolate.

“Only a small daily treat of dark chocolate may substantially increase the amount of antioxidant intake and beneficially affect vascular health,” they said.