Prince Charles faced embarrassment today after an advert for his herbal remedies was banned as misleading.

Tinctures made by the Prince's Duchy Originals brand have already been described as "outright quackery" by a medical expert.

Now the Advertising Standards Agency has reprimanded the company for a promotional email which wrongly claimed the potions, which are added to water and drunk, could cure ills. It is the first British product range to fall foul of new European regulations governing alternative medicines.

The Prince's passion for complementary medicine is said to have led to the product being launched this year.

The email claimed the Duchy Herbals Echina-Relief Tincture "offers natural relief from cold and flu symptoms". It said other products in the range, which include echinacea, hypericum and detox tinctures, provide "alternative and natural ways of treating common ailments such as colds, low moods and digestive discomfort".

One of the recipients wrote to the ASA challenging whether the claims could be proved and after an investigation the watchdog upheld the complaint. It ruled: "We considered that the ad suggested the products had scientifically proven benefits for treating colds and low moods. Because they did not, we concluded that the ad was misleading."

In a statement, the company said: "Duchy Originals accepts the Advertising Standards Authority's decision. The breach was completely unintentional and, as soon as we were alerted to it, we changed the wording and put in place checks to ensure that all future wording is fully compliant with the code."

In March, Charles was accused of "exploiting a gullible public" by putting his name to the detox treatment in the same range. Professor Edzard Ernst, an authority on complementary medicine, said the £10 Duchy Herbals Detox Tincture relied on "superstition and quackery" rather than science. He nicknamed the organic range "Dodgy Originals".

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