Teenage schoolchildren who use e-cigarettes dramatically increase their chances of graduating to tobacco, research has shown.
A study of 14 and 15-year-olds from 20 English schools found a "robust association" between vaping and a higher probability of cigarette smoking.
Children who had never smoked but had tried an e-cigarette were nearly four times more likely to smoke at least one tobacco-filled cigarette within a year than those who had avoided vaping.
Expert opinion is divided on whether e-cigarettes can act as a young person's gateway to tobacco and other drugs.
The devices, which deliver a nicotine "hit" without the dangerous chemicals contained in tobacco, are widely accepted as a safer option for people who already smoke.
Social psychologist Professor Mark Conner, from the University of Leeds, who led the new research, said: "The findings suggest that among the teenagers who had never smoked, the use of e-cigarettes was a strong predicator that within 12 months they would have tried a conventional cigarette.
"It is impossible to say if these young people were just experimenting with cigarettes or were becoming more regular smokers."
A total of 2,836 adolescents aged 14 and 15 were surveyed for the research, published in the journal Tobacco Control.
The vast majority of the children were non-smokers, but a third had experimented with e-cigarettes.
After a year, 34% of those who had never smoked but had tried vaping admitted to smoking at least one "real" cigarette. In comparison, just under 9% of children who had avoided both e-cigarettes and tobacco went on to smoke.
Tellingly, the research showed that children were much more likely to make the jump from e-cigarettes to tobacco if they had no friends who smoked.
This challenges the idea that the vapers would have tried tobacco anyway, whether or not e-cigarettes were available.
Co-author Professor Sarah Grogan, from Manchester Metropolitan University, said: "Adolescents who have used e-cigarettes and who initially have no friends who smoke, may be at particular risk of starting to smoke cigarettes.
"This is particularly interesting as it runs contrary to the suggestion that adolescents who try e-cigarettes would have been likely to try smoking anyway due to factors such as peer pressure from friends who smoke.
"Further work is now needed to understand fully the mechanisms behind this effect."
The evidence also showed that e-cigarettes heightened the chances of teenagers who already had a history of smoking increasing their tobacco consumption.
The researchers speculate that vaping may have the effect of "normalising" a smoking habit, or lead to nicotine addiction, although this is not backed up by scientific evidence.
Since the research began, a new generation of vaping devices has come on to the market that more closely mimic cigarettes, the researchers pointed out. They called for further work to investigate the impact of these devices on smoking risk among young people .
Professor Kamran Siddiqi, another member of the team from the University of York, said: "Our study highlights the value of regulating marketing and sale of e-cigarettes to adolescents.
"The UK has introduced strong regulatory measures in this regard. It is important to enforce these measures effectively and remain vigilant by closely monitoring e-cigarette use in minors."
It has been illegal since October 2015 for retailers in the UK to sell e-cigarettes or vaporising liquids to anyone under 18.
Rosanna O'Connor, director of drugs, alcohol and tobacco at Public Health England, said: "While young people's experimentation with e-cigarettes has been going up, smoking rates have been falling fast. In US studies claiming similar findings, the actual numbers of young people who go from vaping to smoking are very small.
"New UK research will be published later this year bringing together multiple UK studies. All these studies find that while experimentation with vaping by young people is not uncommon, regular use is rare with the great majority being current or ex-smokers."
Robert West, Professor of Health Psychology at University College London, said: "The authors of this study correctly warn readers that it cannot show a causal connection between using an e-cigarette and later smoking.
"In the UK and the US, it seems unlikely that e-cigarette use by young people is causing more of them to smoke because smoking rates in this age group now are declining at least as fast as they were before e-cigarettes started to become popular."