Their use has grown rapidly, with an estimated 1.3 million people using them in 2013.
Currently, e-cigarettes are subject to general consumer protection Iaws.
Until recently, the status of e-cigarettes with regard to helping people quit smoking has been less than clear.
Some people use them to help stop smoking, but they haven’t yet been regulated in the same way as products like nicotine patches. Other people use them as a way to carry on smoking while avoiding cigarette smoke.
The EU is updating the Tobacco Products Directive so that electronic cigarettes below a certain nicotine threshold can be sold with health warnings. Above this nicotine threshold, e-cigarettes will have to be authorised as medicinal products.
E-cigarettes are easily bought in the UK, in shops and online.
During 2014, the government is planning to make the sale of e-cigarettes illegal to under-18s. The same legislation will make it illegal for adults to buy electronic cigarettes and normal cigarettes for children.
An e-cigarette contains three components: a battery, an atomiser and a cartridge containing nicotine suspended in propylene glycol and water. When the user sucks on the e-cigarette, the liquid in the cartridge is heated so that some of it evaporates. This vapour delivers nicotine into the user's lungs.
There is no smoke, but some nicotine vapour is released into the surrounding air as the user exhales.
Some e-cigarettes have an indicator light at the end which glows when the user is inhaling to give an added touch of realism.
Are e-cigarettes regulated?
E-cigarettes are not covered by legislation banning smoking in public places.
It is legal to 'smoke' an e-cigarette in a public place because there is no burning and no smoke is emitted - only odourless vapour.
However, they may still be subject to bans in some places. For example, the train operator ScotRail has banned staff and passengers from using e-cigarettes on its trains, stations or depots. The firm says one reason is their use may unsettle other passengers and make people think that smoking real cigarettes is allowed.
The medicines regulator MHRA says the UK wants to ensure that electronic cigarettes meet appropriate standards of safety, quality and efficacy to help reduce the harms from smoking.
The regulator is encouraging companies to voluntarily submit medicines licence applications for electronic cigarettes as medicines.
E-cigarettes will be subject to the same advertising restrictions as tobacco products.
In January 2013, the Advertising Standards Authority banned a claim that the vapour inhaled and exhaled when using a type of e-cigarette is "a completely harmless vapour". The ASA considered the claims that the product was not harmful had not been substantiated due to an absence of adequate evidence.
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