"Consumption of a normal chocolate bar was associated with a change in stiffness of the blood vessels," said Professor Matthew Walters, who led the study.
"Our data is consistent with a direct effect of chocolate on the brain blood vessels.
"It raises the possibility that there is a direct effect of some component of the chocolate on blood vessels. This is plausible because of the flavonoid molecules contained in chocolate.
"We think a reduction in stroke risk may be caused by chocolate changing how brain blood vessels behave."
The beneficial flavonoids, found in the cacao plant and others, are antioxidants that contribute to the prevention of heart disease.
However, chocolate also has a high sugar and fat content which can cause obesity - a definite risk factor for strokes.
Tom Solomon, professor of neurology at Liverpool University, said: "We have to take the findings with caution."
- DAILY MAIL