A new strain of the sexually transmitted disease gonorrhoea has become resistant to antibiotics, international research shows.
Analysis of the bacterium that causes gonorrhoea found a new variant which is very effective at mutating.
Scientists from the Swedish Reference Laboratory warn that the infection could now become a global threat to public health.
New drugs to delay the spread of the infection are needed, experts say.
The first case of antibiotic-resistant gonorrhoea was found in Japan.
By analysing this new strain of neisseria gonorrhoea, called H041, researchers identified the genetic mutations responsible for the new strain's extreme resistance to all cephalosporin-class antibiotics.
Cephalosporins are used to treat a wide variety of bacterial infections. They are also closely related to the penicillins.
A team of researchers will present its findings at a conference run by the International Society for Sexually Transmitted Disease Research in Canada.
Dr Magnus Unemo, from the Swedish Research Laboratory for Pathogenic Neisseria, said it was an alarming and predictable discovery.
"Since antibiotics became the standard treatment for gonorrhoea in the 1940s, this bacterium has shown a remarkable capacity to develop resistance mechanisms to all drugs introduced to control it.
"While it is still too early to assess if this new strain has become widespread, the history of newly emergent resistance in the bacterium suggests that it may spread rapidly unless new drugs and effective treatment programs are developed."
Prevention not cure
Rebecca Findlay, from the Family Planning Association, said it was a worrying sign.
"Prevention becomes more important because we know antibiotics won't always work. Gonorrhoea can affect people of all ages and everyone should be now focusing on looking after their sexual health."
Dr David Livermore, director of the antibiotic resistance monitoring laboratory at the Health Protection Agency, said that the cephalosporin antibiotics used in the UK are still effective for treating gonorrhoea.
"But our lab tests show that the bacteria are becoming less sensitive to these cephalosporins, with a few treatment failures reported. This means that we are having to change the type of cephalosporin that is used and to increase the dosage.
"The worry is that we will see gonorrhoea becoming a much more difficult-to-treat infection to treat over the next five years.
"Prevention is better than cure, especially as cure becomes harder, and the most reliable way to protect against STIs - including resistant gonorrhoea - is to use a condom with all new and casual partners."
Gonorrhoea is one of the most common sexually transmitted diseases in the world.
Some 50% of women infected with gonorrhoea have no symptoms. The same is true of 2-5% of men.
When symptomatic, gonorrhoea is characterised by a burning sensation when urinating and can cause discharge from the genitals.
If left untreated, gonorrhoea can lead to serious and irreversible health complications in both women and men.