Mouthwash may seem a beneficial, or at least harmless, addition to a daily tooth brushing routine.

But a new study suggests that swilling with anti-bacterial fluid could be killing helpful microbes which live in the mouth and protect against obesity and diabetes.

While mouthwash is supposed to target the bacteria which cause plaque and bad breath, in fact, it is indiscriminate, washing away beneficial strains.

Researchers at Harvard University found that people who used mouthwash twice a day were around 55 per cent more likely to develop diabetes or dangerous blood sugar spikes, within three years.

Although previous studies have found that poor oral hygiene can lead to health problems elsewhere in the body, it is the first research to show that seemingly positive practices can have unexpectedly negative consequences.

Kaumudi Joshipura, professor of epidemiology at Harvard School of Public Health, said: “Most of these antibacterial ingredients in mouthwash are not selective.

“In other words, they do not target specific oral bacteria-instead, these ingredients can act on a broad range of bacteria.”