20pc of French bottled water has traces of pesticides and drugs

They are sold as being cleaner, healthier and purer than the water that spouts from the average French tap.

Now, however, an investigation has discovered traces of pesticides and prescription drugs - including a medicine used to treat breast cancer - in almost one in five brands of bottled water on the shelves of France's supermarkets.

While scientists insist the contamination is minuscule and the water remains safe, consumer groups are warning of a "potential cocktail effect" for drinkers, and say the findings raise serious environmental concerns.

The study was conducted by the consumer magazine 60 Millions de Consommateurs and a non-governmental group specialising in global water issues, Fondation France Libertes.

Researchers analysed 47 brands of bottled water widely available in French shops, and found that 10 contained "residues from drugs or pesticides".

"The biggest surprise was the presence of tamoxifen, a synthetic hormone used in the treatment of breast cancer," wrote the magazine. It reported finding traces of the powerful prescription drug in the popular brands Mont Roucous, St-Yorre, Salvetat, Saint Armand and the Carrefour discount label Celine Cristaline.

It added that the quantity was minute but "enough for us to question the purity of the original produce imposed by regulations covering mineral water".

Traces of the prescription drugs buflomedil and naftidrofuryl, known as vasodilators and used to dilate arteries in those suffering with high blood pressure, were found in Hepar and Saint Armand mineral waters.

Molecules from pesticides banned in 2001 were found in bottles of Vittel, Volvic, Cora and Cristaline.

After the mineral water companies contested the results, the magazine commissioned a second round of tests, which confirmed the first results.

"It's true the micropollutants found were present in very small quantities, but the range of them raises concerns about a potential cocktail effect," 60 Millions de Consommateurs reported. "This is serious enough to call for a much bigger study," it said.