MPs have begun an inquiry into the health effects of e-cigarettes to see how the law could address "significant gaps" in existing research.
The number of conventional smokers in the UK is continuing to fall, with the Office for National Statistics and Public Health England finding there were 7.6 million in 2016 – a decline of 4% since 2010.
However, the use of e-cigarettes has risen to an estimated 2.9 million, up 414% from 700,000 in 2012.
Smoking is believed to cost the NHS roughly £2bn a year, although some warn it could be as much as £5bn, and approximately 80% of the packet price for cigarettes is made of taxation designed to counteract this cost.
However, such taxes are not attached to e-cigarettes, and the impact of the product is unknown, say MPs.
The Royal College of Physicians issued a report last year stating that public health policy should encourage tobacco users to switch to a substitute nicotine product.
It said that "the hazard to health arising from long-term vapour inhalation from the e-cigarettes available today is unlikely to exceed 5% of the harm from smoking tobacco".
However others have warned that e-cigarettes may be equally as harmful as tobacco smoking, although there is little research supporting this claim at the moment.
Norman Lamb MP, the chairman of the Science and Technology Committee, said "there are still significant gaps in the research guiding [e-cigarettes] regulation and sale.
"They are seen by some as valuable tools that will reduce the number of people smoking 'conventional' cigarettes, and seen by others as 're-normalising' smoking for the younger generation.
"We want to understand where the gaps are in the evidence base, the impact of the regulations, and the implications of this growing industry on NHS costs and the UK's public finances."
New rules for e-cigarettes were introduced by the EU's Tobacco Products Directive in May 2016, angering many businesses and users at the perceived EU over-reach away from market regulation and into the arena of healthcare.
The MPs said they will be considering the implications of Brexit on e-cigarette regulation.
The UK Vaping Industry Association said: "We absolutely welcome the inquiry… and we are pleased that vaping is attracting so much attention inside Parliament.
"This will be an excellent opportunity to robustly interrogate the science behind e-cigarettes, and firmly establish vaping as the best way to change the lives of smokers in the UK."