An outbreak of a "concerning" strain of gonorrhoea has been reported in the north of England.

Fifteen cases have been reported where the sexually transmitted infection is highly resistant to azithromycin - the drug most commonly used to treat the disease.

Instances of this particular strain were previously rare, according to the British Association for Sexual Health and HIV.

The outbreak began in Leeds six months ago – but now, cases have been reported in patients from Scunthorpe, Macclesfield and Oldham.

Experts fear there could be more undetected cases, especially as some of the patients had sexual partners from other parts of England.

The association added: "Public Health England is concerned that the effectiveness of current frontline dual therapy for gonorrhoea will be threatened if this resistant strain continues to spread unchecked."

PHE's sexually transmitted bacteria reference unit is currently examining the strains involved.

Gonorrhoea is the second-most common bacterial STI in the UK after chlamydia - and overall, there were almost 35,000 cases of gonorrhoea reported in England last year.

Most cases affect people under the age of 25 - but about 10% of men and almost 50% of women who contract the infection never experience symptoms. 

Jan Clarke of the British Association for Sexual Health and HIV warned that this outbreak needs to be taken very seriously, and that "we are really skating on thin ice as far as treating gonorrhoea is concerned at the moment."