Children who smoke even a single cigarette at an early age are twice as likely to take up the habit later in life, even if they spend several subsequent years not smoking, a study has found.

Scientists argue that this "sleeper effect", which can be triggered by periods of stress or depression, means it is important to prevent teenagers from trying cigarettes even once.

Jennifer Fidler, of the Cancer Research UK Health Behaviour Unit at University College London, followed the smoking habits of more than 2,000 children for five years from the age of 11 at 36 schools in south London.

She said that teenagers who had tried smoking just once in previous years should be considered as new targets for anti-smoking messages.

"This is the first study that shows that a very brief experimentation with cigarettes is predictive of later smoking despite a period of non-smoking in between - it's a dormant vulnerability.

"We were able to show that those students who had reported smoking just once were at an increased risk of taking up smoking by the age of 14, even if they had reported no smoking behaviour in the intervening years," she said.

Of the 260 children who at age 11 said they had tried smoking just once, 18% were smokers at age 14. By comparison, only 7% of children who at age 11 said they had never smoked had started smoking by age 14.

Dr Fidler's team found that the sleeper effect was still present after the usual factors that influence whether someone takes up smoking - ethnicity, gender, social deprivation, and whether a person's parents smoke - were taken into account. The results are published today in the journal Tobacco Control.

Why this effect is present is unclear, although it is known that nicotine affects the brain's reward centres, which release chemicals to tell the body it is doing something enjoyable.

"It's plausible that the reward pathway change makes it more likely on further experimentation that the experience might be more rewarding," said Dr Fidler. That altered reward pathway might easily be triggered years later by something in the school environment, stress or depression.

Alternatively, having a cigarette early on could break down the barriers that prevent teens from smoking, such as fear of adults or concern over how they might look to their peers.

The researchers said that the new study could better inform health professionals, who, in their campaigns to prevent the uptake of smoking, should be additionally targeting a new group of children. "It's a group that might be missed in smoking intervention studies or attempts to prevent smoking," said Dr Fidler. "Perhaps they don't consider themselves smokers and don't appear to be vulnerable to smoking."

She added that focusing on preventing children from trying even one cigarette might be more important than had previously been thought.

"Those children who report having tried smoking, albeit very briefly, are still at increased risk and shouldn't be ignored by people aiming to prevent smoking as they grow older."

Jean King, director of tobacco control at Cancer Research UK, said: "This study is particularly important because, in 2004, 14% of 11-year-olds and 62% of 15-year-olds in England said they had experimented with cigarettes."

Dr Fidler said her study did not imply that all pupils who tried an early cigarette were likely to become smokers. "It isn't the case that you're definitely going to become a smoker. But it has perhaps not been thought in the past that if you've tried one cigarette then you are at risk, even though you might have had a number of years where you haven't been smoking."