Public health messages should be targeted specifically at this neglected group, suggest researchers who carried out the work. The men are a distinct group who have sex with other men, but are not open about their sexuality.
They tend to mix with, and acquire infection from, each other and not from openly gay men. They are unlikely to mix in the same social venues as openly gay men and are not likely to disclose that they have sex with other men.
Fear of stigmatisation, rejection or prejudice can stop this group, who include bisexual and non-gay-identified men, from disclosing their sexuality. They are less likely to receive prevention messages and access the same healthcare as others and as a result may be less aware of their HIV risk.
Researchers at the University of Edinburgh used a national archive of anonymous data to study patterns of HIV transmission. They analysed the genetic code of virus samples from more than 60,000 HIV-positive people in the UK.
Because the genetic information of the virus changes rapidly over time, by finding people whose virus was more similar, scientists were able to create networks of linked infections to see how the virus had spread.
Earlier work from the same group suggested that 6 per cent of men who claimed to be heterosexual at the time of diagnosis had actually become infected through sex with men, not women.
There is little evidence of them spreading the infection to openly gay men or heterosexual women. HIV attacks the body's immune system and, left untreated, makes it difficult fight infections.
Early diagnosis and access to effective treatments allow a near normal and healthy life and prevent onward transmission. Men who have sex with men are the group most at risk from HIV and account for half of those living with the virus in the UK, but they tend to be diagnosed and receive treatment at earlier stages of the disease.
In contrast, heterosexual males remain the group least likely to visit sexual health clinics and are often diagnosed late, when their immune system has already been damaged. The study, published in The Lancet HIV, was funded by the United States National Institutes of Health.
Professor Andrew Leigh Brown, of the School of Biological Sciences, who led the research, said: "Nondisclosed men who have sex with men are more likely to be infected by each other than by openly gay men, and less likely to be aware of their risk. The finding shows that public health messages should be targeted specifically at this neglected group. It also shows that large-scale studies of health data can be carried out without risk to individual privacy."