Teenagers who use e-cigarettes are more likely to take up smoking a year later, research suggests.
Never-smokers who tried e-cigarettes were almost three times more likely to smoke cigarettes a year later compared with those who had never had an e-cigarette.
Just under a third (31%) of more than 2,300 students had used e-cigarettes when they were first questioned in 2013, the study published in the journal Tobacco Control found.
A year later, this figure had risen to 38%.
Most (98%) of those questioned in the first wave of the survey had heard of e-cigarettes, and more than two-thirds (68%) considered them to be healthier than smoking.
The study comes as charity Cancer Research UK said the Government should make the tobacco industry pay for the damage it causes.
It said cuts to public health funding mean local stop smoking services are closing, but industry should "cough up" for such services.
By charging tobacco firms around 1p per cigarette sold in the UK, an extra £500m could be raised, it said.
According to Cancer Research UK's analysis, smoking continues to kill more than 100,000 people in the UK every year.
The tobacco industry makes a profit of more than £4,000 for every UK death caused by tobacco.
Internationally, it makes around £30 billion in profit.
Alison Cox, Cancer Research UK's director of cancer prevention, said: "For too long the tobacco industry has had an easy ride, making money without having to spend a single pound on the damage its products cause.
"It continues to profit from selling a highly addictive and lethal product that causes illness and death. Tobacco companies make billions of pounds every year, so we'd like to see them using their profits to keep stop smoking services open and fund advertising campaigns to help people quit.
"At a time when health budgets are stretched, this is a simple solution to a lethal problem. We urge the Government to make the industry cough up."
The study on teenagers and e-cigarettes was among youngsters living in Hawaii, with an average age of just under 15 years.
Of those who had never smoked e-cigarettes or tobacco at the start of the study, one in 10 had tried e-cigarettes a year later, while 2% had experimented with cigarettes.
Further analysis showed only those teenagers who used a high number of e-cigarettes in 2013 were classed as "regular" smokers of cigarettes a year later.
The authors said: "This suggests that e-cigarette use among adolescents is not without behavioural costs.
"These findings should be considered for policy discussions about the availability of e-cigarettes to adolescents."
In England in 2014, fewer than one in five 11 to 15-year-olds (18%) said they had smoked at least once, according to survey of more than 6,000 pupils - the lowest level recorded since the survey began in 1982.
However, more than a fifth (22%) had used e-cigarettes at least once, including most pupils who smoked cigarettes regularly (89%).
One in 10 (11%) of children who never smoked regular cigarettes had tried e-cigarettes.
Professor Kevin Fenton, national director for health and wellbeing at Public Health England, said: "In the UK, regular use of e-cigarettes among young people is almost entirely limited to those who have already smoked.
"Reassuringly, while e-cigarette use has increased rapidly, rates of youth smoking have fallen to the lowest ever recorded.
"Unlike much of the US, in England it is an offence to sell e-cigarettes to under-18s or to buy e-cigarettes for them and already restricted advertising is set to tighten further with new EU regulations later this year."
Linda Bauld, chairwoman in behavioural research for cancer prevention at Cancer Research UK, said the study did not prove that e-cigarettes provided a "gateway" to regular cigarette smoking.
She said: "Any assumptions that one leads to the other are not supported, as is the case with previous studies.
"In particular in this paper, more frequent use (compared to experimentation) of both e-cigarettes and tobacco at baseline and follow up was low and did not significantly increase - around 8% for e-cigarettes and 4% for tobacco at both time points.
"This study does not provide evidence that e-cigarettes are a gateway to regular smoking in teenagers.
"It is important that ongoing research on this topic is conducted, including in the UK, where e-cigarette age of sale laws already exist and significant marketing restrictions will be introduced from May this year."