Sexually transmitted infections among children and teenagers have doubled in the past 10 years, according to a report published yesterday.

More than 1.3 million people under 20 were diagnosed with an STI in 2001, the Office for National Statistics (ONS) said. The figures showed young people were among the most at risk of contracting a sexual disease.

Women under 20 now have the highest rates of chlamydia, which can cause infertility, among all age groups.

Genital warts, the most common STI, has increased by 15 per cent among teenage women in the past decade.

A third of women diagnosed with the condition in 2001 were under the age of 20, compared with 10 per cent of men in the same age group. The report said: "The sexual health of adolescents in the UK is poor.

"It is likely that an increase in risky sexual behaviour has contributed to sexual health outcomes and unwanted pregnancy among young people,"

Despite millions of pounds being poured into reducing rates of teenage pregnancies, the ONS report shows the strategy is having little success. Pregnancies among 13- to 15- year-olds have remained the same over the past 10 years, at 10 per 1,000 girls.

The report also paints a poor picture of the overall health of children in Britain today.

One in 10 children aged five to 15 have a clinically recognisable mental disorder. Obesity is also on the rise, with 20 per cent of boys and 27 per cent of girls now overweight. In the five years from 1995 to 2000, child obesity increased by 2 per cent among girls and 3 per cent among boys.

The proportion of children who spend less than one hour a week exercising has more than tripled, from 5 per cent in 1994 to 18 per cent in 1999.

The report also says that more than a third of children have unhealthy gums.

Asthma diagnoses have tripled since the 1970s, while British children have among the highest rates of eczema and allergies in the world.

The report adds that huge inequalities still exist between the richest and poorest children in society.

Children from families in the lowest social class are three times more likely to have a mental health problem than those from professional families.