More than 2,000 15-year-olds were infected with chlamydia last year after having under-age, unsafe sex, part of a rising incidence of sexually transmitted infections (STIs) among children.

Detection rates for chlamydia have increased more than 40-fold since 2003 when a national screening programme was introduced, new figures show.

A total of 2,020 teenagers screened positive for the infection while below the age of consent in 2008, compared with 1,176 cases the previous year and just 52 in 2003.

The Government said that the rise was due to more teenagers being tested, but the Liberal Democrats, who obtained the figures in answer to a parliamentary question, accused ministers of complacency about children’s sexual health.

Norman Lamb, the party’s health spokesman, said: “The number of youngsters contacting STIs is very disturbing. Children must be informed about the risks involved in sexual relationships and taught how to be safe.

“The Government has slashed public health spending over recent years. This short-sightedness is putting a whole generation at risk of a sexual health crisis.”

Separate figures from genito-urinary medicine clinics show that the number of diagnoses of all STIs among under-16s in England rose by 58 per cent from 2,474 cases in 2003 to 3,913 in 2007.

The biggest increase was in cases of chlamydia, the most common sexually transmitted infection, which rose by 90 per cent, with genital herpes up by 42 per cent and genital warts by a third.

Among under-16s in England, there were 224 cases of gonorrhoea, six of syphilis, 215 of herpes and 762 of genital warts in 2007.

The National Chlamydia Screening Programme has been phased in since April 2003 with testing in community contraceptive clinics and further education colleges, and more recently through postal testing kits and pharmacies.

About one in 12 people under 25 who are tested are found to have the infection, which often lacks visible symptoms but can have serious consequences such as ectopic pregnancies and infertility.

The programme carried out 359,858 tests in 2007-08, 29 per cent of which were on men and teenage boys.

The Terrence Higgins Trust, the sexual health charity, called for improved sex education in schools to help lower rates of STIs and teenage pregnancies.

Lisa Power, head of policy at the charity, said: “All young people — whether they’re sexually active or not — need to know the facts about safer sex, where they can get condoms and how to use them.

“We need to accept that having a legal age of consent will not stop some young people from having sex; what we can do is make sure that those who do are equipped with all the information they need to protect themselves and their partners from STIs and unwanted pregnancy.”

The Department of Health said: “The National Chlamydia Screening Programme has been rolled out throughout the NHS since 2003. The programme has helped us to screen an increasing number of people for chlamydia. Since 2008, all PCTs have been reporting to the programme, which accounts for the recent increase in reported cases."

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