Cases of an antibiotic resistant sexually transmitted infection have been detected in London after initially being found only in the north, Public Health England (PHE) said Sunday.

Urging people to practice safe sex, the government health agency gave a grim warning that if the current cases of a so-called "super gonorrhoea" become resistant to all forms of antibiotic there is currently no new drug available and the infection could become untreatable.

The safe-sex call comes as an increase in antibiotic resistant cases of the super gonorrhoea continues, a further sign of the very real threat of antibiotic resistance to the ability to treat infections, said PHE.

A national alert was sounded in November, 2014, when the first case was detected in the Yorkshire city of Leeds, with doctors discovering one of the main treatments had become useless against it.

This total number of cases confirmed in England, between November 2014 and April 2016, has now increased to 34, but the number of men and women with the infection could well be much higher as there are often no signs a person is infected.

"PHE continues to monitor and investigate gonorrhoea cases that are highly resistant to the antibiotic 'azithromycin'," said the agency.

"Cases first emerged in the north of England in November 2014. Microbiologists and sexual health doctors in England are being notified that since September 2015 further cases have been confirmed in the West Midlands and in the South of England," added the agency.

Dr Gwenda Hughes, Consultant Scientist and Head of the Sexually Transmitted Infections (STI) Section at PHE, said Sunday: "We continue to maintain an enhanced level of surveillance to identify and manage cases of high-level azithromycin resistant gonorrhoea."

"Since September 2015, 11 cases have been confirmed in the West Midlands and in the South of England, 5 of which were in London. Cases to date have been confirmed in both heterosexual men and women and in men who have sex with men," said Hughes.

"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," explained Hughes.

"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," Hughes warned.