One in five sexually active girls gets pregnant by 18

The findings from the Youth Cohort Study and the Longitudinal Study of Young People in England also show that by the age of 18 eight out of 10 teenagers were sexually active.

Among those who had been pregnant, 18 per cent had been pregnant twice and three per cent had been pregnant three times.

Other findings show that the groups most likely to become sexually active at 14 or under were those who were Neet (not in education, employment or training) at 18 and those eligible for free school meals.

Simon Blake, national director of sexual health charity Brook, said: "We must continue doing what we know works to improve teenage pregnancy; improving access to sexual health services, good-quality sex and relationships education in all schools and the community, and supporting parents to talk to their children about relationships."

However, a study released by Ofsted this week shows that sex education is weak in around a quarter of schools. The study into personal, social, health and economic education (PSHE) found that too often teachers lacked the training and skills to talk discretely to young people about issues such as sex.

Diana Whitmore, chief executive of teenage pregnancy prevention organisation Teens and Toddlers, said: "Young people want sex education to focus much more on relationships and emotion, not just how to put a condom on."