Dangerous binge drug-taking behaviour in London is fuelling a surge in HIV infections among gay men, according to medical research.
The Lancet medical journal reported that the level of new HIV diagnoses among gay men in the city had risen by 21 per cent from 2011 to 2012.
High-risk drug practices in the London gay scene, including smoking, injecting, or snorting of party drugs such as crystal methamphetamine and mephedrone, often in combination with sex parties, were cited as one of the causes of the statistics.
The number of HIV infections rose from 1,420 to 1,720 in English capital, while the rest of the country's figures dropped by 4 per cent.
The percentage increase may be greater still, because the authority which collects the figures adjusts them later, and the 2011 figure has recently risen from 1,296 to 1,420.
Authors of the Lancet article, Tony Kirby and Michelle Thornber-Dunwell, attributed the sharp rise in HIV in the gay community to risk-taking behaviour at drug parties.
In a report from January this year, the same authors said the use of crystal methamphetamine was on the rise, which put men who have sex with other men at higher risk of infections.
"London's 24-hour gay scene is world famous, with clubbing that goes on all weekend and beyond.
"Many of London's drug-using men who have sex with men (MSM), and also lesbians, and their heterosexual male and female friends often congregate at post-club parties known as chill-outs where drug-taking continues until drugs, money, energy, or all three run out.
"There are also many sex parties where MSM congregate at homes for group sex."
The authors said participants have multiple partners, sometimes over several days, and did not use condoms. Needles for drug use were often shared, contributing to the infection rate.
Some of the 2011-12 rise could also be attributed to increased HIV testing, with 43,404 tests in London in 2012, up 17 per cent on the 2011 figure of 37,010.
Public Health England head of HIV surveillance Valerie Delpech said the rise in London was concerning.
"The good news is that increased HIV testing in recent years accounts for some of this rise, however the scale of the increase seen shows us that high rates of transmission are ongoing."
"There is anecdotal evidence from drug and alcohol clinics in London that recreational or club drug use amongst gay men is a growing issue, which warrants further investigation.
"PHE, in collaboration with London clinics, is initiating enhanced behavioural surveillance amongst gay men likely to have acquired their infection in the 6 months before diagnosis, to explore this issue."
Drug clinics available were "oversubscribed and struggling to cope," the Lancet article said.