Sales of the herbal remedy echinacea, which is purported to boost the immune system, are predicted to hit £30m by the end of the year, up by 30 per cent on last year, amid growing medical controversy over whether it works.

It is the best-selling herbal remedy in health food shops, according to the National Association of Health Stores (NAHS). "There are occasional studies disputing its effectiveness but it depends a lot on what type you use and how well it has been stored," said the NAHS chairman, John McKee.

The medical establishment in the UK refuses to endorse echinacea, despite it being the most researched plant in the modern herbal world.

Among of the plant's first users were the North American Plains Indians, who used it as a remedy for various ailments including snake bites, toothaches, sore throats and even smallpox. A recent US study found that of 399 volunteers given the common cold virus, those treated with echinacea were just as likely to develop cold symptoms as those given a placebo.