A study suggests that high doses of the herbs can damage reproductive cells and prevent sperm fertilising eggs.

They might even damage eggs and sperm, it found.

The research appeared in the journal Fertility and Sterility.

Herbal treatments

Clinical tests have shown that St John's Wort can help treat depression and anxiety.

Echinacea is said to help prevent colds

Gingko biloba is marketed for boosting memory and is thought to help patients with Alzheimer's disease.

Echinacea is sold to bolster the immune system against colds and flu.

A team of scientists at Loma Linda University School of Medicine in California investigated the effects of the herbs on fertility.

They said St John's Wort in particular impaired the ability of the sperm to penetrate the egg. It also seemed to cause genetic mutations in the sperm.

Alan DeCherney, editor of the journal, said: "This is a very important study that could provide important information to patients suffering from infertility.

"The growing popularity of these herbal products means we must examine all their possible side effects."

Hamster eggs

The researchers tested the herbal preparations under laboratory conditions. Healthy human sperm usually can penetrate a hamster egg, but will not fertilise it.

There was no change in performance at very low doses, but at higher doses St John's Wort prevented the sperm achieving penetration.

Echinacea and gingko also impaired penetration.

Sperm exposed to St John's Wort for more than a week were damaged, the researchers added.