According to the largest ever clinical study of echinacea, the herbal remedy can prevent colds and is of most benefit to people who are prone to them.

Researchers found that taking three daily doses of the common remedy for four months reduced the number of colds.

The duration of the illness suffered by patients also went down by an average of 26%.

According to the results of tests on 750 people, the treatment also cut the number of recurrent colds suffered by those with weak immune systems or a history of catching several bouts each year by 60%.

Several previous studies, including an overview of evidence by the highly respected Cochrane Library, had suggested that echinacea could soothe symptoms and cut colds short, but there was only limited evidence it could prevent the illness from ever taking hold.

The most recent major paper into the therapy, by the American College of Physicians, had found that it did not prevent colds or significantly reduce the length or severity symptoms.

But the new study by experts from the Cardiff University Common Cold Centre suggested that taking Echinaforce, a common form of the herb extract, could not only reduce the risk of colds but reduce the amount of paracetamol patients took while ill.

The research, which was part-funded by A. Vogel, the Swiss manufacturers of Echinaforce, was published in the peer-reviewed journal Evidence-Based Complementary and Alternative Medicine.

It was primarily designed to test the safety of the treatment, and found that it caused no adverse side effects in the participants, who were all over the age of 18.

Echinacea is extracted from the Eastern Purple Coneflower, which is found in North America, and has long been used as a herbal remedy for the common cold.

It is purported to work by fighting viruses, which cause up to 95% of all colds and flu, and studies suggest it can also boost weak immune systems if swallowed.

Patients mixed 25 drops of Echinaforce or a placebo with water and held it in their mouths for 10 seconds before swallowing it, three times per day over a four month period.

Those who took the treatment suffered 149 bouts of illness compared with 188 in the placebo group, a difference described by researchers as “borderline significant”, but the total number of days spent with flu was reduced from 850 to 672, a “highly significant” change.

Recurring infections were cut from 100 episodes in 43 patients to 65 episodes in 28 patients, a difference of 59%, the authors wrote.

Roland Schoop, a medical researcher for Bioforce, the British arm of A. Vogel, and one of the study’s authors, told the Daily Telegraph: “We were actually pretty amazed when we found this 26% difference in cold episodes.”