E-cigarettes are contaminated with lung-damaging bacterial toxins of the type found in waste-incineration plants, scientists have discovered.

An investigation by Harvard University found that more than one in four of the increasingly popular products harboured bugs known to cause asthma, inflammation and reduced lung function.

In addition, the study revealed that sweet fruity flavours, which are particularly popular among teens, were most likely to carry the toxins.

Previous research has shown that chemicals linked to severe respiratory disease are found in common e-cigarette flavours.

However, this is the first time scientists have found potentially harmful biological substances in the devices.

Public Health England (PHE) campaigns strongly for cigarette smokers to switch to e-cigarettes, arguing the products are, at worse, only five per cent as harmful as traditional smoking.

This stance is coming under mounting criticism, however, from scientists who say it is too early to know the true dangers of vaping.

The Harvard team examined 75 popular e-cigarette products, namely single-use cartridges and e-liquids for refillable devices.

They found that 27 per cent contained traces of endotoxin, a microbial agent in agricultural and industrial settings, and that 81 per cent contained traces of glucan, which exist in the cell walls of most fungi.