The group came under fire after five people complained to the Advertising Standards Authority (ASA) about a media campaign which recommended drinking four cups a day as part of a healthy diet.
ASA upheld several complaints on the grounds the adverts were misleading.
Despite the UK Tea Council (UKTC) submitting nearly 100 scientific studies backing up tea's health potential, the ASA said it had not seen evidence to " firmly substantiate" any health benefit in drinking four cups of tea per day and said the campaign "exaggerated" the health benefits.
Food health claims are a hot topic at the moment as companies who wish to make a health, nutritional or disease related claim about a food will have to comply with EU legislation.
Although no approved wording or accepted health claims have been approved, it seems that advertising regulators are already tightening up on health claims in preparation, and food companies should think carefully before making a health claim.
Under regulation (EC) No 1924/2006, which came into force in the UK from 1 July 2007, any food product claiming to have a health benefit must meet a list of European Commission approved wording and be supported by scientific evidence.
Although the regulation came into force this month in the UK, the European Commission is not expected to agree a list of approved literature until the end of the year.
The scope of the changes will not just affect food packaging, but any content including websites associated with the food.
The British Dietetic Association supports the drinking of tea as part of a balanced diet because of its flavonoids content.
However, the group warns that excessive amounts of tea should be avoided by children because tea is also a diuretic and contains caffeine.
One of the posters from the UKTC said: " Four cups a day can contribute to a diet rich in antioxidants which could help to protect your body against the damaging effects of free radicals".
The campaign was also criticised for being misleading because it implied the advice was part of a health campaign by a Government department or similar authority and did not make clear that the ads were by an industry body, the ASA said.
UKTC said studies showed that tea polyphenols entered and were retained for a time in the body, and were available in sufficient quantities to make an active contribution.
It also said they had selected conditional language in its campaign.
ASA consulted an expert who pointed out that two of the studies submitted by UKTC had been commissioned by the tea industry and were not independent.
She said: " The evidence relating to any health benefit arising from tea consumption was promising, but inconclusive."
She concluded that the research suggested that drinking four cups of tea per day would lead temporarily to increased antioxidant levels, but the evidence was not yet available to confirm that small increases in serum antioxidant levels had beneficial health effects.
In conclusion the ASA said:" We considered that we had not seen substantive evidence to demonstrate that the antioxidant potential realised from the consumption of four cups of tea per day could have any effect on free radical activity."
UKTC, which had been planning to run the posters in October, said it was " completely mystified " and will appeal against the ruling.
Executive chairman William Gorman said the ASA appeared to have had a " complete u-turn " as the council had consulted with the Committee of Advertising Approval before running the ads.
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. "
ASA did not uphold the complaint that adding milk and sugar affected the bioavailability of antioxidant compounds.