Health reporter, BBC News
There were 18,710 cases of gonorrhoea reported in 2007
Fears have been raised over strains of drug-resistant gonorrhoea that have emerged in England and Wales.
Tests on samples of the sexually transmitted infection have shown high levels of resistance to a commonly used antibiotic called azithromycin.
Health Protection Agency researchers said the finding was a "significant public health concern".
Gonorrhoea is the second most common STI in the UK and young people are most at risk of infection.
Early symptoms of the infection can be mild and people may be unaware they have caught it.
But if left untreated there can be serious consequences, especially in women who can develop pelvic inflammatory disease which can damage the fallopian tubes, causing infertility.
The HPA set up a surveillance programme in 2001 to specifically monitor whether antibiotics were remaining effective against the infection.
It first spotted six cases in 2007 which were resistant to azithromycin - five from Liverpool and one from Cardiff.
Further analysis published in the Journal of Antimicrobial Chemotherapy shows there has been a general trend towards antibiotic resistance in recent years, which means treatment will very likely not work, even if the dose is increased.
Surveillance in the US and Scotland has highlighted similar "drifts" towards resistance, the researchers said.
Azithromycin is not currently recommended as a first choice therapy for people with gonorrhoea but it is supposed to be a fall-back drug if the standard ones stop working.
The researchers said resistance could have occurred because the drug was being used against official advice.
Another potential source of resistance is that low doses of the antibiotic are commonly given for chlamydia.
Some of those patients are likely to also have undiagnosed gonorrhoea, which would not be fully treated, paving the way for resistance to develop.
Study leader Dr Stephanie Chisholm, said antibiotic resistance is able to develop very easily in the organism that causes gonorrhoea and antibiotics ciprofloxacin, tetracycline and penicillin have already become useless because of the problem.
If high-level resistance to azithromycin spreads further, this is one less treatment option available for the future, she added.
"At present there is no evidence to show that these strains will cause any different symptoms to the strains that are currently circulating.
"Despite this resistance, appropriate antibiotic treatment should cure all uncomplicated gonococcal infections.
"It is vital that those who are infected seek treatment, because if left untreated complications can occur such as pelvic inflammatory disease, chronic pelvic pain, ectopic pregnancy or infertility."
She advised everybody to use condoms with new and casual partners and that gonorrhoea could be transmitted by unprotected oral sex.
Vicky Sheard, policy officer for the Terrence Higgins Trust said: "Many STIs are very easy to treat but they don't clear up on their own.
"If left untreated some STIs can cause more serious health problems.
"If people have had unprotected sex it's important they get checked and get the right treatment as soon as possible."