The team of researchers from the University of Strathclyde made 40% of human skin cancer tumors disappear using the compound, in a laboratory study.
Green tea has long been suspected of having anti-cancer properties, and the extract, called epigallocatechin gallate, has been investigated. However, this is the first time researchers have managed to make it effective at shrinking tumours.
Previous attempts to capitalise on its cancer-fighting properties have failed because the scientists had used intravenous drips, which failed to deliver enough of the extract to the tumors, themselves.
The Strathclyde team devised an alternative "targeted delivery system," piggy-backing the extract on proteins that carry iron molecules, which cancer tumors readily absorb.
The laboratory test on one type of human skin cancer showed 40% of tumors disappeared after a month of treatment, while an additional 30% shrank.
Dr. Christine Dufes, a senior lecturer at the Strathclyde Institute of Pharmacy and Biomedical Sciences, who led the research, said, "These are very encouraging results which we hope could pave the way for new and effective cancer treatments.
"When we used our method, the green tea extract reduced the size of many of the tumours every day, in some cases removing them altogether. By contrast, the extract had no effect at all when it was delivered by other means, as every one of these tumours continued to grow.
"This research could open doors to new treatments for what is still one of the biggest killer diseases in many countries."
She added, "I was expecting good results, but not as strong as these." Dufes said population studies had previously indicated that green tea had anti-cancer properties, and scientists had since identified the active compound as epigallocatechin gallate.
However, the Strathclyde researchers were the first to delivery it in high enough doses to tumors to have an effect.
She added, "The problems with this extract is that when it's administered intravenously, it goes everywhere in the body, so when it gets to the tumors, it's too diluted.
"With the targeted delivery system, it's taken straight to the tumors without any effect on normal tissue."
Cancer scientists are increasingly using targeted delivery to improve results, relying on the many different "receptors" that tumors have for different biological substances.
In this instance, the scientists used the fact that tumors have receptors for transferrin, a plasma protein which transports iron through the blood.
The results have been published in the journal Nanomedicine.
The "ultimate objective" was a clinical trial in humans, but Dufes said that was some way off. "We have got to optimize the delivery system and therapeutic effect first," she said.
Dr. Julie Sharp, from Cancer Research U.K., commented, "A few studies have shown that extracts from green tea may have some effect on cancer cells in the lab, but this has not yet been backed up by research in humans."
She added, "It's far too soon to say if enjoying a cup of green tea has any wider benefits in combating cancer, but we know that a healthy balanced diet can help to reduce the risk."
It will, however, be years before scientists can say for sure if green tea has provided them with a new cancer wonder drug.