The number of diagnoses of the sexual transmitted infection gonorrhoea has increased by an “unprecedented” 25 per cent in the past year, the Health Protection Agency has revealed.

Gonorrhea, the second most common bacterial STI in the UK, is becoming increasingly resistant to antibiotic treatment and Britain faces “the very real danger of the untreatable gonorrhoea in the future”, health experts said.

There were 20,965 new cases of gonorrhoea diagnosed in England in 2011, up from 16,835 in the previous year. Overall, diagnoses of STIs were up by two per cent in the UK in 2011, reversing a small decline observed in the previous year.

Although the dramatic increase in gonorrhoea cases was partly attributed to more sensitive testing, the HPA said that too many people were still putting themselves at risk, with gay men and young heterosexual adults were the groups causing the greatest concern. Instances of gonorrhoea were up by 61 per cent among men who have sex with men and young heterosexual adults, between 15 to 24-years-old, represented 57 per cent if all new gonorrhoea diagnoses.

Dr Gwenda Hughes, head of STI surveillance at the HPA, said: “We anticipated some increase in diagnoses due to improvements in testing in recent years, but not on the scale seen here,” she said. “Too many people are putting themselves at risk of STIs and serious health problems by having unsafe sex.”

Dr Hughes said that in the past five years laboratory testing of the gonorrhoea bacterium had shown greatly increased resistance to the main drugs used to treat the infection.

Gonorrhoea can have serious effects if left untreated. Young women with gonorrhoea can develop pelvic inflammatory disease (PID), which can cause chronic abdominal pain and also lead to infertility.

New diagnoses of syphilis were up by 10 per cent to 2,915 and herpes was up five per cent to 31,154. Although the number of new cases of Chlamydia was down by two per cent to 186,196, Dr Hughes said this was most likely a reflection of fewer young people getting tested.

Sexual health campaigners warned that public health authorities had “taken their foot of the pedal” on informing and warning the public about the dangers of STIs and said that the upcoming transfer of responsibility for public health to cash-strapped local authorities risked worsening the problem.

“These figures are a salutary lesson,” said Lisa Power, policy director of the Terrence Higgins Trust, the UK’s leading sexual health charity. “Sexual health services have had a much lower profile in recent years and it’s demonstrable what happens when you let up on informing the public about the risks. We’ve been working hard with local authorities and our experience is that the services on offer will incredibly variable. There will be a postcode lottery for sexual health unless services are improved across the board.”