Health experts have warned that there is a "very real threat" that gonorrhoea could become incurable.
The sexually transmitted disease (STI) - the second most common in the UK - has developed a resistance to the only antibiotic that has been used to treat it for the last five years.
The Health Protection Agency (HPA) is recommending that clinics now stop prescribing the widely-used antibiotic cefixime because it is not completely effective in almost one in five cases.
Tests from samples and taken from patients have shown reduced susceptibility to cefixime in nearly 20 per cent of cases in 2010, compared with just 10 per cent in 2009.
Describing the situation as an "alarming decrease", the HPA is recommending that doctors now use a combination of two drugs instead: ceftriazone, given by injection, and azithromycin, a pill given orally.
HPA experts say the STI has been easy to treat for the last 70 years, but now the organism that causes the infection, Neisseria gonorrhoeae, has adapted to become resistant to four types of antibiotic: penicillin, ciprofloxacin tetracyclines, and now cefixime.
"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," said Professor Cathy Ison, a gonorrhoea expert at the HPA.
"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 warned.
This highlights the importance of practising safe sex, as if new antibiotic treatments can't be found, this will be the only way of controlling this infection in the future."
According to HPA figures, there were 16,145 new diagnoses of gonorrhoea in 2010, a 3 per cent increase from 2009. The highest rates of gonorrhoea are seen in women aged 16-19 and in men aged 20-24 years.