For a bank holiday, it's not exactly a cheerful message. This weekend, there will be 1,440 new cases of chlamydia, 572 cases of genital warts and 207 cases of gonorrhoea contracted by people having unprotected sex.

In an attempt to warn people in a party mood that they need to take extra precautions, NHS officials have produced figures of the number of people they predict will pick up a sexually transmitted infection (STI). Health workers are also distributing 50,000 condoms around pubs and clubs as a practical measure to make it easier to reduce the risk.

The statistics, extrapolated from the rising annual toll of infections, are part of a new strategy to make the under-25s think twice before having unprotected casual sex. The NHS 'Sex Lottery' campaign has produced a website encouraging people to send saucy postcards to their friends which contain jokes about gonorrhoea and syphilis. It includes images of men whose groins can be 'scratched' by the computer mouse to see whether they have picked up an infection.

Figures earlier this summer showed a leap in the number of such diseases. Cases of chlamydia, the most common such infection and one which can cause infertility in women, rose 9 per cent last year. The number of people infected with syphilis rose by a massive 28 per cent.

The Department of Health said it was keen to get the safe sex message out over the bank holiday weekend. 'With recent statistics showing an increase in rates, the Sex Lottery has launched a dedicated summer campaign to help raise awareness of the risks of unprotected sex,' said a spokeswoman.

'The message is clear this bank holiday weekend - don't play the Sex Lottery. Use a condom.'

The new approach was welcomed by the FPA, formerly the Family Planning Association. A spokeswoman said: 'Raising awareness about safe sex and distributing free condoms is great.'

But there has been criticism of the way the government has reacted to the emerging infections epidemic. People who suspect they may have an infection can wait weeks to be seen at an NHS genito-urinary clinic. The number of people visiting these clinics has soared in recent years and the NHS is struggling to cope.

The FPA recently revealed that 16 per cent of clinics had actually cut their opening times compared with two years ago. In addition, just two new clinics had opened since 2002.

'If someone tries to go to a clinic and doesn't get seen, the chances are they will not be back,' the FPA spokeswoman said. 'They carry on having sex and can transmit their infection to others.'