Gay men are suffering serious harm and are in danger of spreading HIV by having unprotected sex while under the influence of illegal drugs, the first British study into the growing popularity of "chemsex" has revealed.

Men who use substances such as crystal meth while having sex are at risk of overdosing, being hospitalised, losing consciousness, having panic attacks or convulsions, serious mental health problems and sexual assault, according to research conducted among one of the country's largest gay populations.

Three-quarters of those interviewed had participated in reckless sexual behaviour while high on drugs, which ran the risk of fuelling the already rising rates of HIV and sexually transmitted infections (STIs) among men who have sex with men (MSM).

The worrying findings are contained in the Chemsex report, commissioned from the London School of Hygiene and Tropical Medicine (LSHTM) by the south London boroughs of Lambeth, Southwark and Lewisham.

Although the study involved in-depth interviews with just 30 men involved in chemsex, the fact that three out of four of them had engaged in sex with a high risk of passing on HIV or another STI has caused concern. Three had been sexually assaulted after passing out, and two others had seen that happening or heard of it happening from friends.

Paul Steinberg, Lambeth's HIV prevention and sexual health commissioning manager, said that the trend towards chemsex is being facilitated by a "perfect storm" of cheap, easily available drugs, a burgeoning number of social and sexual networking apps and websites that enable men to arrange to meet for sex at private parties.

"There has been a change in gay cultural, social and sexual networks. In the 1980s and 1990s gay men would meet in bars, clubs and community centres. But with the rise of the internet and social media apps, there has been a shift towards a technology- and home-based cultural scene", says Steinberg.

Adam Bourne, the LSHTM academic who led the research, says: "A vulnerable section of society is using new drugs in new ways that are putting them at serious risk."

The report found that the three most commonly used drugs in chemsex were crystal methamphetamine, GHB/GBL and mephedrone, though some used cocaine and ketamine too.

"They allow you to have sex for much longer, which may mean you choose to have sex with more people in that time. From an HIV transmission and STI transmission point of view, having sex for longer and with more partners means there's a higher likelihood of transmission, because of skin damage on and around the sexual organs, particularly if it's condomless sex, which is often a feature of chemsex", explains Bourne.

More than a quarter of those interviewed, all of whom were among the 13 participants who were HIV positive, had decided to have unprotected sex with men they believed were the same status. Worryingly, the report added: "Nearly a third of men found it difficult to control their behaviour while under the influence of drugs and engaged in HIV/STI transmission risk behaviour, which they subsequently regretted. These were often men who had pre-existing problems negotiating safer sex, which were exacerbated by the presence of drugs."

In 2012, the three councils commissioned the community services arm of nearby Guy's and St Thomas's NHS trust to set up a healthy gay business and community initiative. It involves an inspection regime for the 15 gay clubs and three saunas in the boroughs – a voluntary code of conduct, in effect. Council licensing officers, GPs, hospital doctors and gay men's sexual health charities are involved, as is Public Health England (PHE). The next stage is to start talking to those behind the apps and websites promoting casual sex. But no one pretends there are easy answers.

"Specialised sexual health services are the best place to help men who have sex with men and other individuals who are regularly taking these high risks," says Jan Clarke, the president of the British Association for Sexual Health and HIV.

The new Chemsex report is helping to shape two forthcoming key documents: the three boroughs' HIV and sexual health strategy 2014-17 for their 25,000-30,000 gay residents, and PHE's national framework for improving the health of MSM.