Companies are marketing the electronic cigarettes as a way of getting round the ban.
And in recent months they have reported a rise in sales with some selling over 1,000 of the Ģ40 starter-packs a month.
While the products do not contain tar, tobacco or carbon monoxide, experts are worried as users inhale a fine heated mist and there is a lack of regulation.
But retailers said they were healthier than normal cigarettes.
However, campaigners including the World Health Organisation, have raised concerns, pointing out there was a lack of knowledge about the products.
Douglas Bettcher, of the World Health Organisation, said there was a "regulatory blackhole" which meant no-one knew just what these products contain.
"We are facing a new product. We do not know what is in these cartridges besides nicotine. What are the effects of heating and vapourising the nicotine and inhaling it?"
And Deborah Arnott, of anti-smoking group Ash, pointed out that many of the products were made in China where quality control was "not very good".
"I think our concern is that we would really like smokers to use safer nicotine products but there is a regulatory gap.
"The advice is to use products that have been tried and tested such as nicotine gums and patches."
Jason Cropper, managing director of the Electronic Cigarette Company, said: "It's a healthier way of smoking. You don't get any of the stuff that's in regular tobacco.
"It's better if people aren't using nicotine in any form, but they're an alternative to using a tobacco-based product and my opinion is they're massively less harmful. I bet my life they're a hundred times healthier than using a tobacco-based cigarette.
"Tests have been done on mice and in the lab and they have shown they are not harmful."
But he said it had not been possible to carry out human trials as they were too expensive.
"Most of these companies selling these are small companies. We do not have the finances to do that.
"It might be an idea if we can get someone to work with us."
He, however, asserted: "I believe this product is a lifesaver. Ultimately it will become the nicotine replacement therapy of choice."