Green tea's ability to fight cancer is even more potent and varied than scientists suspected, research suggests.
Scientists already know that green tea contains anti-oxidants which may have a protective effect against cancer.
But now they have discovered that chemicals in the tea also shut down a key molecule which can play a significant role in the development of cancer.
The molecule, known as the aryl hydrocarbon (AH) receptor, has the ability to activate genes - but not always in a positive way.
Tobacco smoke and dioxins, in particular, disrupt the functioning of the molecule and cause it to trigger potentially harmful gene activity.
The researchers, from Rochester University, found that two chemicals in green tea inhibit AH activity.
Both chemicals are similar to compounds called flavonoids, which are found in broccoli, cabbage, grapes and red wine, and which are also known to help prevent cancer.
Researcher Professor Thomas Gasiewicz said: "Green tea may work differently than we thought to exert its anti-cancer activity.
"It's likely that the compounds in green tea act through many different pathways."
The Rochester team showed that the chemicals shut down the AH receptor in cancerous mouse cells.
Early results indicate the same is true in human cells.
However, the scientists say that the results in the laboratory do not necessarily translate to everyday life as the crucial factor is how green tea is broken down inside the body.
In addition, there are a lot of differences between various types of green tea.
Dr Julie Sharp, a science information office at Cancer Research UK, said: "This research describes additional properties of green tea that may be beneficial but which have yet to be tested properly.
"The causes of cancer are complex and both diet and our genetic make-up act together to influence our risk of developing the disease.
"Cancer Research UK is currently involved in a large-scale study of diet and health that is researching the eating habits of over half a million people in 10 European countries to try and help unravel this complexity."
Research has also suggested that green tea may help to reduce the risk of rheumatoid arthritis and to lower cholesterol levels.
The research is published in the journal Chemical Research in Toxicology.