Children in the schools which ran the programme were 25 per cent less likely to take up regular smoking / Reuters

The most important health warning that parents can give their children – don't smoke – is best delivered by their friends, researchers have found.

Training children who are popular at school to educate their peers about the dangers of smoking could cut the number who take up the habit by more than a fifth, a study showed. If the same technique were used nationwide, the number of children aged 14 and 15 who take up smoking could be cut by 43,000 a year, researchers estimate.

It is unclear whether young people smoke because their friends do or whether those who choose to smoke associate with others who are similarly inclined. What is clear, according to the researchers from the universities of Bristol and Cardiff, is that peer influence can be protective, if it can be effectively harnessed.

To do this they launched a two- year study in 59 schools in the West Country involving 11,000 children aged 12 to 13. In half of the schools, the children were asked to nominate the most influential pupils in their year group and these were trained as "peer supporters". The remaining schools acted as a control group.

In two days of training outside school, teachers advised on the risks of smoking and the economic benefits of stopping, and taught negotiating skills. This was followed by four sessions in school. The peer supporters included existing smokers who were told they could be trained if they gave up cigarettes.

For the next 10 weeks, the peer supporters were asked to talk to their friends about the benefits of not smoking, in the hope it would persuade them to stop. It worked. Children in the schools which ran the programme were 25 per cent less likely to take up regular smoking immediately after it ended than those in the control schools.

The effect was sustained but diminished over time, with a 23 per cent reduction in smoking after one year and 15 per cent after two. Saliva samples were taken and questionnaires handed out to check whether pupils had been smoking. The results are published in The Lancet.

The programme was popular with students and staff, with a more than 90 per cent response rate and no schools dropping out during the trial, suggesting it could successfully be extended. The schools had widely differing catchment areas, and the scheme worked as well with smokers among the pupils as with those who had never smoked. Saliva testing showed most pupils accurately reported their smoking.

The authors point out that stopping young people from smoking successfully stops them developing nearly all of the diseases associated with it. Once smoking is started, however, evidence shows that it is harder for poorer people to give up.

"Increasing resources for prevention in adolescence rather than focusing on cessation could help to avoid widening health inequalities," they say.