UK doctors are being told the antibiotic normally used to treat gonorrhoea is no longer effective because the sexually transmitted disease is now largely resistant to it.
The Health Protection Agency says we may be heading to a point when the disease is incurable unless new treatments can be found.
For now, doctors must stop using the usual treatment cefixime and instead use two more powerful antibiotics.
One is a pill and the other a jab.
The HPA say the change is necessary because of increasing resistance.
Tests on samples taken from patients and grown in the laboratory showed reduced susceptibility to the usual antibiotic cefixime in nearly 20% of cases in 2010, compared with just 10% of cases in 2009.
As recently as 2005, no gonorrhoea bacteria with reduced susceptibility to cefixime could be found in the UK.
The bacterium that causes the infection - Neisseria gonorrhoeae - has an unusual ability to adapt itself and has gained resistance, or reduced susceptibility, to a growing list of antibiotics - first penicillin itself, then tetracyclines, ciprofloxacin and now cefixime.
The World Health Organization recommends that the first-line antibiotic used is changed when treatment failure in patients reaches 5%.
But for cefixime, the change is being made pre-emptively, owing to the alarming rise in resistance that is emerging.
Prof Cathy Ison, a gonorrhoea expert at England's HPA, said: "Our lab tests have shown a dramatic reduction in the sensitivity of the drug we were using as the main treatment for gonorrhoea. This presents the very real threat of untreatable gonorrhoea in the future.
"We were so worried by the results we were seeing that we recommended that guidelines on the treatment of gonorrhoea were revised in May this year, to recommend a more effective drug.
"But this won't solve the problem, as history tells us that resistance to this therapy will develop too. In the absence of any new alternative treatments for when this happens, we will face a situation where gonorrhoea cannot be cured."
She said patients who refuse the jab will be offered oral antibiotics instead.
She added: "This highlights the importance of practising safe sex, as, if new antibiotic treatments can't be found, this will be only way of controlling this infection in the future."
After genital chlamydia, gonorrhoea is the second most common bacterial sexually transmitted infection in the UK.
According to HPA figures, there were 16,145 new diagnoses of gonorrhoea in 2010, a 3% increase on 2009 when there were 15,606.