More than one in three gay men with HIV have unprotected sex, a survey says.
The University College London poll of 2,640 men in Manchester, Brighton and London also revealed a fifth of gay men without HIV do the same.
Researchers said it was worrying, and called for a renewed push to discourage risky behaviour, the Sexually Transmitted Infections journal said.
But experts said evidence suggested many men with HIV were having unprotected sex with others with HIV.
The latest research was carried out between 2003 and 2004 in gay clubs, bars and saunas.
The rate of HIV infection was highest in Brighton, at almost 14%, and lowest in Manchester, at 8.6%
Across the entire sample, one in three men who was HIV positive did not know they had the infection - a figure which tallies with previous research.
This was despite the fact that over two-thirds of these men said they had been to a sexual health clinic within the past year.
Some 18% of HIV negative men and 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.
Lead researcher Dr Danielle Mercey said: "We have to renew our efforts to ensure people with HIV get early diagnosis and also look to curb risky behaviour.
"This is particularly important for younger men who are more likely to demonstrate risky behaviour in much the same way as they are more likely to drive fast or take drugs.
"It is only by early diagnosis and safe sex that we will reduce the rate of HIV."
About a third of new HIV diagnoses are gay men, with over 7,450 new cases being identified in 2005 - slightly up on the previous year.
But other experts questioned how risky the behaviour was.
Michael Carter, of the Aidsmap support group, said: "The results of the survey are entirely realistic, but before we condemn the figures we have to see it in context.
"Many of these men with HIV will be having sex with other men with HIV."
But he admitted this could have other health risks such as sexually transmitted infections and the small risk of a "super infection" - re-infection with a different strain of HIV.
Will Nutland, head of health promotion at the Terrence Higgins Trust agreed that many of the men with HIV were having sex with other people with the same status.
He added: "The new research also backs up other evidence which demonstrates the need for ongoing, targeted HIV prevention work with gay and bisexual men in the UK.
"The number of gay men with undiagnosed HIV infection is not reducing."