Figures from the Public Health Laboratory Service show that contractions of all STDs were up by 9% between 1996 and 1997.

Chlamydia cases rose 20%, gonorrhoea 5% and and genital warts 8%, with the sharpest increases for chlamydia and genital warts among 16 to 19-year-olds.

2,500 HIV cases a year

The service fears the figures may reflect a greater willingness to take risks with respect to HIV in younger sections of the population.

A study by the Schools Health Education Unit appeared to confirm this trend when it found that concern about Aids is falling among children aged 12 to 15.

This is despite statistics that show new infections are running at a constant rate of 2,500 a year.

A PHLS spokesman said the safe sex message needed to be heavily emphasised.

Gay risk

"If we are to prevent a rise in HIV infections in young people, they must be told about safer sex before they start having unsafe sex," he said.

"Research in the United States shows how successful targeted prevention can be.

"Between 1991 and 1997 there was a 23% increase in the number of young Americans using condoms when they last had sex."

The PHLS said it was particularly keen to see safe sex used by the UK's most at-risk group from HIV - gay men.