Tea drinkers have been told to give milk a miss if they want to help their hearts.

Studies show that drinking tea can help protect against cardiovascular disease. But now researchers believe that adding milk counteracts any beneficial effects. When black tea is drunk on its own, cardiovascular function improves.

The research, published in the European Heart Journal, involved studying 16 healthy post-menopausal women. They were given either 500ml of black tea, black tea with 10 per cent skimmed milk or boiled water as a control.

They drank it on three separate occasions but refrained from drinking tea for four weeks both before and after the study.

All the women were given a croissant as a standardised breakfast while they drank the tea.

In a healthy artery, blood vessels are able to relax if the blood flow increases. This is called flow-mediated dilation (FMD).

The researchers from the Charite Hospital Universitatsmedizin Berlin in Germany, measured FMD levels in the forearm using ultrasound. They did this before tea was drunk and at intervals afterwards.

The study said: "Black tea significantly improved FMD in humans compared with water, whereas addition of milk completely blunted the effects of tea."

The researchers believe that proteins in milk, called caseins, could be to blame for the negative effect.