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Blends from the Asda Smartprice, Tesco Value, Morrisons Value and Sainsbury's Basics range were found to have an average of six milligrammes (mg) per litre.

In comparison, branded black blends such as PG Tips, Twining's and Typhoo were found to have an average of 3.3 mg per litre, the study found.

Pure blends Oolong and Pu'er teas were found to have the lowest concentrations of fluoride - an average of 0.7 mg per litre.

Laura Chan, who carried out the study for her PhD at the University of Derby with Professor Aradhana Mehra and Professor Paul Lynch, said: ''The tea plant, Camellia sinensis, is a fluoride accumulator, with mature leaves accumulating most of the fluoride.

''When tea is harvested, these 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 the manufacture of higher grade and speciality tea products.

''Although fluoride is considered an essential micro-nutrient for human health, in the prevention of tooth decay and promotion of healthy bone growth, excess fluoride in the diet can have detrimental effects.

''Dental fluorosis, the mottling of tooth enamel, and skeletal fluorosis, pain and damage to bones and joints through calcification, can occur.

''People may be drinking excessive volumes of tea in addition to other dietary sources of fluoride and may not realise these potential health implications. Indeed, there have been cases, in both the UK and the USA, of skeletal fluorosis in individuals who drank more than the average amount of economy tea.

''All tea products should be considered as a main source of fluoride in the diet, and we would urge supermarkets and manufacturers of tea to consider stating fluoride concentration as part of the nutritional information found on food packaging.''

The study is published in journal Food Research International.