One in nine young people under 25 is testing positive in the national screening programme for the sexually-transmitted disease chlamydia, suggesting that the fertility-threatening infection is even more widespread than feared.

The rate is higher in men than women - nearer one in eight of those tested - but few men are putting themselves forward for checks, raising fears that women who are diagnosed and treated with antibiotics are risking reinfection. Only one in four primary care trusts in England is screening young people but half of England's 4.25 million 15-24 -year-olds should be screened within two years, according to government targets.

Many of those infected show no symptoms despite the fact that the disease can lead to severe health problems, particularly in women, and is easily spread through unprotected sex. This prompted the screening programme in England, and other parts of the UK are considering following suit.

Health experts say testing and treating a person for uncomplicated chlamydia may cost £10-£15 a time, as against the £4,000 of one cycle of IVF treatment sought by infertile couples. Tests are offered at family planning services, GP surgeries, higher education institutions, military camps, and a number of other services. Some of those volunteering for the tests are being sent home-testing kits and in at least one area, people are being sent their test results by text message. Boots is offering a free service in its pharmacies across London from next week in a two year experiment which may be extended if successful.

The primary care trusts so far involved in the programme screened 60,698 young people between April 2004 and March 2005, only 12.5% of them men. Among women, 10.9% tested positive; the figure was 11.9% in men. Among 15-year-olds 3% of boys and 9% of girls tested positive.

Caroline Flint, public health minister, said yesterday: "When we see these figures it sets alarm bells ringing ... There is no point in women coming forward to be screened and getting treatment and then going back to a male partner who will reinfect them."

The government plans a further high-profile media campaign in the new year urging young people to protect themselves better against sexually-transmitted infection and unintended pregnancies.

Disease and risks

· Chlamydia is the most common sexually transmitted infection in Britain, diagnosed in 103,673 people at genito-urinary clinics in 2004

· A voluntary screening programme in England has already found 6,689 cases in its second year, more than one in 10 of those tested

· Half all infected men and seven in 10 women have no symptoms

· If left untreated, 10-40% of women develop pelvic inflammatory disease, with infection in the uterus, ovaries and fallopian tubes. Up to one in five of these will become infertile, and the risk of ectopic pregnancy greatly increases

· Uncomplicated chlamydia is easily treated by antibiotics, either in a single dose or twice daily for a week