An outbreak of highly drug-resistant gonorrhoea has been detected in the north of England, triggering a national alert.
Fifteen cases have been detected by Public Health England (PHE) so far, the British Association for Sexual Health and HIV said.
Reports to PHE's sexually transmitted bacteria reference unit (STBRU) of cases of the infection being highly resistant to the azithromycin drug have previously been rare, it added.
The outbreak, which was first detected in Leeds in March, has since spread, with cases reported in patients from Macclesfield, Oldham and Scunthorpe.
All of the cases involve heterosexual patients, and some people have reported partners from other parts of England.
The association said: "An outbreak control team meeting has been convened and STBRU are currently performing next generation sequencing on these strains to better understand the molecular epidemiology."
It added: "PHE is concerned that the effectiveness of current front-line dual therapy for gonorrhoea will be threatened if this resistant strain continues to spread unchecked."
The number of infections detected may seem small, Peter Greenhouse, a consultant in sexual health based in Bristol said, but added there could be more undetected cases.
He told the BBC: "This azithromycin highly-resistant outbreak is the first one that has triggered a national alert.
"It doesn't sound like an awful lot of people, but the implication is there's a lot more of this strain out there and we need to stamp it out as quickly as possible.
"If this becomes the predominant strain in the UK we're in big trouble, so we have to be really meticulous in making sure each of these individuals has all their contacts traced and treated."
There were almost 35,000 cases of gonorrhoea reported in England last year - it is the second most common bacterial sexually transmitted infection in the UK after chlamydia.
The majority of cases affect people under the age of 25. Around 10% of men and almost half of women with the infection do not experience symptoms.