Children who try just one cigarette are twice as likely to take up smoking as those who have never tried it, a study funded by Cancer Research UK suggests.
Just one cigarette can lead to a habit, the study suggests
The 2,000-pupil study, in Tobacco Control, found this was the case even after a gap of three years or more.
It is the first to find a smoking "sleeper effect" - where desire remains years after the first cigarette.
Cancer Research UK said anti-smoking campaigns should focus on preventing children trying even one cigarette.
Around 14% of 11-year olds and 62% of 15-year olds have smoked a cigarette.
Prevention messages should also be targeted at children while they are still at primary school, warned the researchers.
Young people in 36 London schools were surveyed every year from the age of 11 to 16.
The 12% of 11-12 year olds who admitted smoking just once were more likely to take up smoking when they were older compared with those who had never smoked, even after a gap of up to three years of not smoking.
The researchers said the "sleeper effect" could be explained in several ways.
Nicotine in a single cigarette may effect pathways in the brain increasing the likelihood that someone will start smoking in response to other triggers, such as stress.
Alternatively, trying a cigarette may break down barriers that might prevent someone taking up smoking - such as fear of being caught or insecurity about how to smoke.
Study leader Dr Jennifer Fidler, a research psychologist at the Cancer Research UK health behaviour unit in London said: "It is known that past smoking behaviour predicts future behaviour and it could take some time to progress from an experimental to regular smoker.
"But this is the first study that shows an early experience with one cigarette leads to smoking several years in the future."
She added: "There are two important messages - firstly it may be more important than previously thought to try and prevent children from trying even one cigarette and, secondly, health professionals and those working in smoking prevention in schools need to be aware that those who have tried one cigarette, but are not smokers, are at risk.
"Prevention efforts might be most effective if focused on pre-secondary school children."
Deborah Arnott, director of anti-smoking charity Ash, said she would not be surprised if further research showed that one cigarette had an addictive effect.
"This is very interesting because other bits of research seem to show there is conditioning of nicotine receptors and it also happens with second-hand smoke because if parents smoke in front of their children, they are more likely to smoke - and there is evidence to show that's an effect of nicotine conditioning as well."
Simon Clark, director of the smokers' lobby group Forest, said: "The idea that one cigarette can have such a profound effect on anyone, including teenagers, seems rather dubious.
"I accept that children who smoke a cigarette at the age of 11 may be more likely to take up smoking within the next few years but, whatever this report says, I suspect that has far more to do with immaturity, a rebellious personality and a lack of parental guidance."