The number of teenagers smoking remains worryingly high, researchers say, with girls twice as likely as boys to take it up.

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A study by Mark Conner, of the University of Leeds, tracked adolescents for six years with questionnaires and breath and saliva tests.

He found that focusing anti-smoking efforts on children between the ages of 11 and 14 did not deter them from smoking later.

When the children were younger, they were given materials on the dangers of smoking and their ability to reject cigarettes. The project seemed to reduce smoking levels.

When researchers returned when many of the children were between 15 and 16 years old, most had not smoked in the last three months.

The previous study also found that figures for regular smoking were very similar for boys and girls at age 11-12, by the time they were 13 or 14, girls were twice as likely to be smokers (16 percent compared with 8 percent).

In the latest work, this pattern was found to continue among the 15-16 age group, with 31 percent of girls and 16 percent of boys regularly indulging.

"It is not clear from our results why girls are more likely to smoke," Connor said. "This is an area we'd like to devote more attention to in the future."