A third of gay men with the HIV virus do not know they are infected, research has suggested.
The findings make stark warnings about HIV in the UK
A team at the Royal Free and University College Medical School gathered saliva samples from 1,206 gay men.
The study in the journal Sexually Transmitted Infections found 10.9% were HIV positive, but in a third of these cases the virus had not been diagnosed.
Experts estimate that around the same proportion of heterosexuals infected with HIV are unaware of their status.
Estimates say that 50,000 people in the UK have been infected with the HIV virus.
Researchers from the Royal Free and University College Medical School interviewed more than 8,000 gay men in London between 1996 and 2000, and noted an increase in risk taking behaviour.
Whereas in 1996, 30% of men said they had unprotected sex in the previous 12 months, by 2000 this had risen to 42%.
Some respondents had never had an HIV test and of those who said that they knew their HIV status, 4% were wrong.
And 16% of men who had a sexual relationship with the same HIV status partners were either incorrect about their diagnosis, or could not be completely certain about it.
Experts stress the need for everybody to practise safe sex to protect themselves from HIV and other sexually transmitted infections.
'Focused health promotion'
The researchers said: "A high proportion of HIV positive men continue to engage in high-risk sexual behaviour after diagnosis, emphasising the need for focused health promotion programmes to reduce the risk of HIV transmission to others."
In the UK, 50,000 people are estimated to be HIV-positive
Earlier this year the Health Protection Agency (HPA) reported a 20% increase
in people being diagnosed with HIV during 2003.
Dr Barry Evans, an HIV expert at the HPA, said increases in people having
unsafe sex were "undoubtedly the main driving force" behind the growing
More than 2,000 new cases among gay men were expected to be recorded once all
reports for 2003 were received, the agency said.
Lisa Power of the Terrence Higgins Trust told the BBC it believed it should be made easier for people to have an HIV test.
She added: "People are having unprotected sex across the board - it's not just gay men.
"We need to get the message across to say that we all need to protect our health - whether or not we think we have a sexually transmitted infection, whether or not we think our partner has - it's important that all of us should practise safe sex."