More than 1,200 under-16 girls reported infections in 1998, a rise of two-thirds since 1990. Among young women aged 16 to 19, cases were up by a third, reaching almost 20,000 in 1998 - the last year for which official figures are available.

Incidences of chlamydia - which can lead to pelvic inflamation and infertility - doubled among the under-16s, with more than 550 girls infected. Cases of genital warts were up by half, reported by 425 girls under 16.

There were many fewer cases of sexual disease reported among boys, and they are rising more slowly. Only 191 boys under 16 reported cases n 1998 - a rise of just 4 percentage points.

The Department for Education and Employment is due to issue new guidelines on sex education. A spokeswoman said that they would make clear that the best way to prevent infection was through delaying sexual activity.

The figures, published by the Department of Health in a written answer in the House of Lords, will be cited by campaigners who claim ignorance is largely to blame for the increase.

Diane Lea, spokeswoman for the Family Planning Association, said: “There is evidence that girls are starting menstruation as early as eight or nine, but sex education lessons do not start until 11. They are not early enough and they are not comprehensive enough.”