Trying e-cigarettes makes children nearly four times more likely to smoke tobacco within a year, research suggests.
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.
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.
The research showed children were much more likely to make the jump from e-cigarettes to tobacco if they had no friends who smoked.
The evidence also showed that e-cigarettes heightened the chances of teenagers who already had a history of smoking increasing their tobacco consumption.
Robert West, Professor of Health Psychology at University College London, said there was no evidence of a "causal connection between using an e-cigarette and later smoking".
He said: "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."
It has been illegal since October 2015 for retailers in the UK to sell e-cigarettes or vaporising liquids to anyone under 18.