Its poster recommended drinking four cups of tea daily to "contribute to a diet rich in antioxidants."
The advertisement said, "Five portions of fruit and veg plus four cups of tea. It all adds up to a healthy diet."
However, after consulting an expert the Advertising Standards Authority (ASA) found no evidence to "firmly substantiate" any health benefit from drinking four cups of tea per day.
It found the poster in breach of three different clauses of the advertising code.
The watchdog also criticised the U.K. Tea Council for making reference to the government's 'five-a-day' campaign which could mislead readers into thinking the poster was government-linked.
An independent expert who advised the ASA said evidence about the health benefits of drinking tea was "promising but inconclusive."
There is "mounting and promising" experimental evidence that tea has a protective effect against cancer and heart disease.
However, laboratory findings have yet to be confirmed through human trials, the expert said.
The ASA said, "We told UKTC (Tea Council) not to imply in future campaigns that there was an established health benefit, in terms of antioxidant potential, to be had from drinking four cups of tea per day."
It also told the Tea Council to avoid implying in future material that its message was linked to the government.
Responding to the investigation, the U.K. Tea Council said tea was rich in plant-derived antioxidants called flavonoids.
It submitted a scientific paper about black tea which suggested drinking three cups per day could help prevent heart disease.
The Tea Council said its advert had not suggested tea could be a substitute for eating fruit and vegetables.
Responding to the ASA's ruling, U.K. Tea Council executive chairman William Gorman said, "We provided the ASA with almost 100 independent scientific research papers, and yet they still turned us down despite acknowledging that the antioxidants in tea are absorbed into the body.
"Many of the papers we presented used the same methodology to show that fruit and veg are good for you, but the ASA effectively told us we'd have had to run clinical trials, normally reserved for medical drugs."
The U.K. Tea Council represents tea-producing countries plus U.K. tea packing and distribution firms.
From the October 8, 2007, Prepared Foods e-Flash