TEA drinkers who opt for cheaper supermarket blends could be at a higher risk of bone and teeth problems, according to researchers.

A study by the University of Derby found cheaper blends contain enough fluoride to put people drinking at least four cups a day over their daily recommended limit, increasing the risk of such illnesses.

Significant differences in fluoride levels were discovered when economy black tea blends from supermarkets Asda, Tesco, Morrisons and Sainsbury's were compared with branded blends such as PG Tips, Twinings and Typhoo, researchers said.

They compared the fluoride levels ingested by someone drinking the average daily intake of tea (four cups or one litre).

In comparison, branded black blends had an average of 3.3 mg per litre, while pure blends Oolong and Pu'er teas had an average 0.7 mg/litre.

Laura Chan, who carried out the study for her PhD, said: "The tea plant, Camellia sinensis, is a fluoride accumulator, with mature leaves accumulating most of the fluoride.

"Older leaves may be used to produce lower quality, stronger teas such as economy teas, whereas the bud and newer top leaves are used in higher grade and speciality tea products.

"We would urge supermarkets and manufacturers of tea to consider stating fluoride concentration on food packaging."

The study is in Food Research International.