But the World Health Organisation and scientists from the London School of Hygiene and Tropical Medicine and the University of Liverpool remain concerned about their safety.

In the new experiment the team used normal epithelial cells, which line organs, glands, and cavities throughout the body, including the mouth and lungs.

The cells exposed to the e-cigarette vapour showed several forms of damage, including DNA strand breaks. The double helix that makes up DNA has two long strands of molecules that intertwine. When one or both of these strands break apart and the cellular repair process doesn't work right, it raises the risk of cancer.

The affected cells were also more likely to launch into apoptosis and necrosis, which lead to cell death.

The scientists tested two types of each e-cigarette: a nicotine and nicotine-free version. Nicotine is what makes smoking addictive. There is also some evidence it can damage cells. The San Diego team found that the nicotine versions caused worse damage, but even the nicotine-free vapor was enough to alter cells. Nicotine free e-cigarettes caused 50 per cent more DNA breaks, while for those containing nicotine the damage rose three fold.