Electronic cigarettes are to be classed as "medicines" under new proposals to tighten up the regulation of nicotine-containing products.
Manufacturers are to face tough new tests before they can sell their e-cigarettes as "licensed products", the Medicines and Healthcare products Regulatory Agency (MHRA) said.
The move will also mean that licensed e-cigarettes can be prescribed by medics to help smokers cut down or quit.
It is estimated that 1.3 million people across the UK use battery-powered e-cigarettes, which can look like real cigarettes but users inhale a mist of nicotine instead of smoke.
When the user sucks on the e-cigarette, liquid nicotine is vapourised and absorbed through the mouth. When they breathe out, a plume of what appears to be smoke is emitted but it is actually largely water vapour.
In the past, experts have said that if all smokers were to use e-cigarettes instead of normal cigarettes, millions of lives could be saved, because they do not contain harmful tar like in real cigarettes. But others have raised concerns about safety and regulation of the products.
Experts said that users could not be certain about the purity of the nicotine contained inside. MHRA research found that nicotine levels can be "considerably different" from the level stated on the label. And the amount of nicotine per product was found to differ from batch to batch - which casts doubt on how useful the products are to people who want to cut down or to stop smoking, a spokesman said.
Now the MHRA will regulate the products so there is a consistent standard across all licensed products sold over the counter. While the regulator is not planning to recall unlicensed e-cigarettes - saying that the benefit outweighs the risk of users turning to real cigarettes - it is hoped that in the long term all products will be licensed.
James Dunworth, co-founder of EcigaretteDirect.co.uk, said: "Electronic cigarettes are currently regulated. What we need is not more regulations, but an enforcement of existing ones.
"Our eliquid is produced from pharmaceutical grade nicotine in laboratory conditions, and has been tested in a UK government approved lab."