Adding milk to a cup of tea can destroy its ability to protect against heart disease, according to research.
A small German study found drinking black tea significantly improved the ability of arteries to relax and expand to keep blood pressure healthy.
But the European Heart Journal paper also found proteins in milk, called caseins, blocked this effect.
It is estimated as many as 98% of UK tea-drinkers prefer milk in their favourite cuppa.
The researchers tested the effects of tea in 16 humans and on rat tissue.
They showed molecules in the tea called catechins helped dilate the blood vessels by producing a chemical called nitric oxide. The caseins in milk prevented this effect by reducing the concentration of catechins in the tea.
"Our results provide a possible explanation for the lack of beneficial effects of tea on the risk of heart disease in the UK", Professor Stangl
Senior researcher Dr Verena Stangl, professor of cardiology at the Charite Hospital, in Berlin, said: "Our results thus provide a possible explanation for the lack of beneficial effects of tea on the risk of heart disease in the UK, a country where milk is usually added."
However, June Davison, cardiac nurse at the British Heart Foundation (BHF), said: "It is difficult to say from this small study the impact of adding a drop of milk to your tea can make.
"The tea break is a great British tradition which provides time to relax with a cuppa in hand.
"Leaving milk out of your tea is far less likely to help protect your heart health than other measures, such as taking regular exercise, avoiding smoking and eating a healthy balanced diet."
But Ms Davison also said the study highlighted the importance of not just thinking about one food in isolation but the effect of the actual interaction between different foods.
Prof Stangl said the study was very complex and so could only be performed on a small number of people.
Professor Andrew Steptoe of UCL's department of epidemiology and public health, who has previously carried out research into the effects of tea on recovery from stress, said that as such studies were very difficult to carry out he was not surprised that this study had been very small.
"There are benefits for tea, with or without milk, so keep on drinking", Catherine Collins
On the results of the study, he added: "We would be interested to know if that sort of effect persists long-term or if it is just an acute effect of tea."
Prof Steptoe also said that as there were about 200 bioactive compounds in tea the apparent effect of milk of vasodilation "does not necessarily mean milk negates the other effects of tea".
Catherine Collins, a dietician and spokesperson for the British Dietetic Society, agreed that tea was a "very healthy drink" and pointed out that drinking it with milk in would boost calcium intake.
She said: "There are benefits for tea, with or without milk, so keep on drinking."
Bill Gorman, chairman of the Tea Council, also said the study was "another very positive piece of research for tea as it's clear that the researchers recognise that tea has significant health effects".