PARIS - Bad news for Britons: adding milk to tea ruins the health benefits of the drink, according to a Germany study.

Tea has complex compounds called polyphenols which are believed to help the arteries to relax or dilate, thus enabling a smoother flow of blood.

Scientists led at the Charite Hospital in Berlin tested black Darjeeling tea on 16 healthy women volunteers aged more than 50, placing an ultrasound probe on their forearm to measure arterial response.

When the women drank half a litre (0.9 of a pint) of tea, their arteries relaxed significantly more than when they drank hot water or tea with milk -- tea in which skimmed milk, comprising 10 percent of the drink's volume, was added.

The results were confirmed in lab-dish tests on rat aorta.

The study, which appears online in the European Heart Journal, points the finger of blame at three casein proteins in the milk. These are thought to adhere to a kind of polyphenols known as catechins, preventing them from carrying out their health-making work.

This could explain why Britain, a nation passionate about tea-drinking but where almost everybody adds milk to their cup, fails to make headway against cardiovascular disease, said researcher Verena Stangl.

The study did not cover green tea, which is widely drunk in East Asia -- without milk.