E-cigarettes come in two parts: in one end there is liquid nicotine, and in the other a rechargeable battery and an atomiser.

When the user sucks, the liquid nicotine is vaporised and absorbed through the mouth.

What looks like smoke is largely water vapour, hence smoking e-cigarettes is often called vaping.

As the e-cigarettes contain nicotine, the user gets the hit that makes smoking addictive, without being harmed by the toxins found in real cigarettes.

However, there is a consensus that they are certainly less harmful than smoking tobacco, which is responsible for 100,000 deaths a year in the UK.

Should smokers use them to quit?

They are not recommended by the NHS yet, but there is some evidence they may help.

A team at the University of Auckland, in New Zealand, last year released the results of the first clinical trial comparing the devices with nicotine patches.

The results, published in the Lancet, showed 7.3% using e-cigarettes had quit after six months compared with 5.8% using patches.

What is more, after six months, 57% of e-cigarette users had halved the number of cigarettes smoked each day compared with 41% among those using patches.

However, the study did not involve enough people - just 657 - to prove definitively which is the better option.