More than one in three HIV positive gay men say they have unprotected sex, reveals a community survey, published ahead of print in the journal Sexually Transmitted Infections.
And almost one in five HIV negative men said that they do the same, the figures show.
The findings are based on a survey of men at gay bars, clubs, and saunas in London, Manchester, and Brighton between 2003 and 2004.
The surveys, which were carried out at 90 venues, involved information on lifestyle, background, and sexual behaviour.
Almost 3600 questionnaires were handed out, 2640 of which were completed. Saliva samples were also requested to find out about HIV prevalence, and 2311 men agreed to provide these.
The rate of HIV infection was highest in Brighton, at almost 14%, and lowest in Manchester, at 8.6%
But the rates of undiagnosed infection was high in all three cities, ranging from one in three in Brighton to more than four out of 10 (44%) in London.
Across the entire sample, one in three men who was HIV positive did not know they had the infection.
This was despite the fact that over two thirds of these men said they had been to a sexual health (GUM) clinic within the past year.
Almost one in five (18%) of HIV negative men and over a third (37%) of HIV positive men said they had had unprotected sex with more than one partner in the past year.
Over the same time period, one in five HIV negative men and four out of 10 HIV positive men said that they had had a sexually transmitted infection.
The authors conclude that despite the availability of treatment and a national policy to promote HIV testing, a significant proportion of infection remains undiagnosed.
They call for renewed efforts to be made to boost diagnosis and curb risk behaviours that encourage onward transmission of HIV and other sexually transmitted infections.