A highly drug-resistant type of “super-gonorrhoea” is spreading across the country, with senior medics warning it may become untreatable.

A powerful strain of the sexually transmitted superbug first seen in the north of England has been found in the West Midlands and the south-east, Public Health England (PHE) said.

The strain is highly resistant to the antibiotic azithromycin, which means medics are relying on a second drug, ceftriaxone, to treat it. But there are no other effective drugs to tackle the strain, raising the prospect of it becoming untreatable if it builds further resistance.

PHE urged people to use condoms with new or casual partners to cut the risk of catching the disease. If untreated, gonorrhoea can result in severe complications and in rare cases lead to infertility or septicaemia.

Dr Gwenda Hughes, the head of PHE’s sexually transmitted infections (STI) section, said: “Fortunately, the current outbreak strain can still be treated with ceftriaxone. Nonetheless, we know that the bacterium that causes gonorrhoea can rapidly develop resistance to other antibiotics that are used for treatment, so we cannot afford to be complacent.

“If strains of gonorrhoea emerge that are resistant to both azithromycin and ceftriaxone treatment options would be limited as there is currently no new antibiotic available to treat the infection.”

PHE said on Sunday there had been 34 confirmed cases since November 2014. Since September 2015, 11 cases have been confirmed in the West Midlands and in the south of England, five of them in London.

At least 16 cases were first detected in northern England, including 12 in Leeds, where the mutated strand was first recorded, PHE said in September.

The strain, which is resistant to first-line antibiotic azithromycin, spread from Leeds to patients in Macclesfield, Oldham and Scunthorpe. Cases have been found in heterosexual men and women, and men who sleep with men (MSMs), PHE said.

The British Association for Sexual Health and HIV issued an alert to clinicians urging them to follow up cases of high-level drug-resistant gonorrhoea and trace their sexual partners.

Its president, Dr Elizabeth Carlin, told the BBC: “The spread of high-level azithromycin-resistant gonorrhoea is a huge concern and it is essential that every effort is made to contain further spread. Failure to respond appropriately will jeopardise our ability to treat gonorrhoea effectively and will lead to poorer health outcomes for individuals and society as a whole.”

There were almost 35,000 cases of gonorrhoea reported in England in 2014 and it is the second most common bacterial sexually transmitted infection in the UK after chlamydia, with the majority of cases affecting people under the age of 25.

Infected patients may experience discharge or pain while urinating, but about 10% of men and almost half of women do not suffer any symptoms.

Concerns have been growing over “untreatable” strains of gonorrhoea, and in 2012, the European Centre for Disease Prevention and Control warned that drug-resistant forms of the STI were spreading across Europe.