Almost one in ten cases of gonorrhoea in England and Wales has developed resistance to a key antibiotic used to treat it, says a report.
Gonorrhoea is a common bacterial infection
The study, carried out in genitourinary clinics in both countries, found that the rate of resistance had more than tripled since last year.
The authors say the antibiotic is no longer effective, and that a new strategy needs to be found urgently.
Gonorrhoea is the second most common sexually-transmitted bacterial infection in the UK.
It causes a painful and unpleasant infection - and in extreme cases can even prove fatal if left untreated.
In addition, the bacteria which cause it, Neisseria gonorrhoeae, can damage fertility in women.
Unlike viral sexually transmitted infections such as HIV or herpes, doctors can prescribe antibiotics which should clear it up quickly.
But experts are becoming increasingly worried about the effectiveness of the best-known of these, ciprofloxacin, since strains of gonorrhoea resistant to it started to emerge in Asia.
An initiative set up to measure the prevalence of resistant strains found that in 2000, only one in 50 strains had this ability.
In 2001, this had increased to 3.1% - but the latest figures, published in the Lancet medical journal, show that the figure has risen to 9.8%.
In some areas things were worse - in the Yorkshire and Humberside area, more than 18% of strains isolated had resistant qualities.
The situation is not as bad as in China and Hong Kong - where 98% of strains have resistance to common antibiotics - but the researchers are alarmed by the finding.
They wrote: "It is a general principle with gonorrhoea that the chosen treatment should eliminate infection in at least 95% of patients, and ciprofloxacin no longer meets this criterion.
"The data suggest that national and local treatment guidelines need to be reviewed urgently."
Dr David Hawkins, from the genitourinary clinic at the Chelsea and Westminster Hospital in London, said that while the figure was alarming, there were still plenty of other antimicrobial drugs that could deal with the infection.
He said: "Most clinics including our own have already modified their practices as a result of this.
"We are fortunate to have identified this early thanks to a good surveillance system."
The number of people catching gonorrhoea in England and Wales is also on the rise, with the number of diagnoses doubling to more than 20,000 between 1995 and 2000.
The rises are highest among older teenagers.
The government published a sexual health strategy in 2001 which aimed for a 25% fall in gonorrhoea cases by 2007.
Misuse of antibiotics has been blamed for the rise in the number of strains of common bugs which have resistance.
A Department of Health, which funded the study, said: "Action is already in hand to alert clinicians to the findings from this study and to alternative drug regimens pending the current review of the national treatment guideline."
He said the department was working with the Health Protection Agency and professional groups to disseminate the information.
Information is also available on the Medical Society for the Study of Venereal Disease (MSSVD) website