A chemical extracted from green tea could help scientists to develop new drugs to fight cancer.
Green tea has been linked with a series of health benefits
Tests by UK and Spanish researchers showed polyphenol EGCG taken from green tea leaves inhibits cancer cell growth.
The effect was seen even at low concentrations, equivalent to drinking two or three cups of green tea a day.
However, the study, published in Cancer Research, also found high concentrations of the chemical may increase the risk of birth defects.
Previous research has suggested that drinking green tea helps to cut the risk of certain forms of cancer.
The latest study found that EGCG binds to a key enzyme - dihydrofolate reductase (DHFR) - that is targeted by established anti-cancer drugs.
This stops the enzyme from triggering the manufacture of new DNA in tumour cells.
It appears to work in the same way as the cancer drug methotrexate - but in practice would probably have fewer side effects.
Professor Roger Thorneley, from the John Innes Centre in Norwich, conducted the research with team from the University of Murcia in Spain.
He said: "This is a very exciting discovery. For the first time we have a clear scientific explanation of why EGCG inhibits the growth of cancer cells at concentrations which are found in the blood of people who drink two or three cups of green tea a day.
"We have identified the enzyme in tumour cells that EGCG targets and understand how it stops this enzyme from making DNA.
"This means we may be able to develop new anti-cancer drugs based on the structure of the EGCG molecule."
Fellow researcher Dr Jose Neptuno Rodriguez-Lopez said: "We decided to look at EGCG because we recognised that its structure is very similar to that of the successful anti-cancer drug methotrexate.
"We discovered that EGCG can kill cancer cells in the same way as methotrexate.
"However, because EGCG binds to the target enzyme less tightly than methotrexate, it should have decreased side effects on healthy cells."
Fewer side effects
Dr Rodriguez-Lopez said the researchers were now using EGCG as the starting point to design and develop effective new anti-cancer drugs that kill tumour cells but inflict less damage on healthy cells.
Previous studies have also linked high levels of green tea consumption around the time of conception and during pregnancy with an increased incidence of neural tube defects such as spina bifida.
These defects are associated with a lack of the key nutrient folic acid.
The scientists said EGCG in green tea would be expected to cause a significant drop in folic acid levels.