Health experts are warning of the "very real threat" that gonorrhoea could become incurable.
The sexually transmitted infection (STI) - the second most common in the UK - has developed resistance to a type of antibiotic that has only been used to treat it for the last five years.
This "alarming decrease" in the effectiveness of cefixime means a new, stronger treatment regime must be put in place, the Health Protection Agency (HPA) said.
In some cases, patients have not responded to treatment on cefixime.
Lab tests showed the infection had a reduced susceptibility in 17.4% of cases in 2010, compared to 10.6% in 2009.
In 2005, there were no cases reported as having reduced susceptibility to the antibiotic.
HPA experts say the STI has been easy to treat for the last 70 years but the organism that causes the infection - Neisseria gonorrhoeae - has an "unusual ability to adapt itself" and has gained resistance to a growing list of antibiotics, from penicillin to tetracyclines, ciprofloxacin and now cefixime.
Sexual health doctors are now being told to use a combination of two drugs - ceftriaxone, a more powerful antibiotic than cefixime, which is delivered by injection, and azithromycin, which is given orally.
Professor Cathy Ison, a gonorrhoea expert at the 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.
"Many patients may feel anxious about having an injection, but this is now the best way of avoiding treatment failure. Patients who refuse the jab will be offered oral antibiotics instead.
"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."
Terrence Higgins Trust's clinical director, Jason Warriner, said: "It's worrying that gonorrhoea has developed resistance to certain drugs in a relatively short period of time. We're pleased to see health experts rapidly bringing forward changes in treatment and continuously monitoring the situation to ensure the infection continues to be treatable.
"Gonorrhoea doesn't always present symptoms so it's vital that people and their partners have regular screenings for it, alongside other STIs, in order to get treated at the earliest possible stage. The only way to prevent STIs is to use condoms."
According to HPA figures, there were 16,145 new diagnoses of gonorrhoea in 2010, a 3% increase on 2009 when there were 15,606.