A third of gay men with HIV do not know they are infected, research reveals today. The growing number of homosexual men having unprotected sex while not knowing their HIV status represents a serious public health concern, researchers said.

Their study, published in the journal Sexually Transmitted Infections, confirms fears that as many as one in three of the UK's estimated 50,000 HIV sufferers are unaware of their infection.

The researchers, from the Royal Free and University College Medical School, questioned more than 8,000 gay men in London's bars, clubs and saunas between 1996 and 2000.

In 2000 they also gathered anonymous saliva samples from 1,206 men to test their HIV status.

About one in nine of the samples (10.9 per cent) was positive for HIV.

In a third of these cases - 43 - the HIV had not been diagnosed.

And of those men who said they knew their HIV status, four per cent got it wrong.

The researchers, led by Julie Dodds and Danielle Mercey, also noted an increase in risk-taking behaviour during their study.

In 1996 about one in three men (30 per cent) said they had had unprotected sex in the past 12 months.

By 2000, this figure had risen to more than four in ten (42 per cent).

The researchers said: "A high proportion of HIV saliva antibody 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."

The team said that of the men having unprotected anal sex, 45 per cent said they only did so with partners with the same HIV status.

But the researchers raised concerns that 16 per cent of men who reported having a sexual relationship with same HIV status partners were either incorrect about their diagnosis or could not be completely certain about it.

"They had either never had an HIV test, reported their status incorrectly or did not know their HIV status," they said.

Overall, the study found that 10.9 per cent of socially active gay men in London in the group they studied were HIV positive.

The researchers concluded: "HIV prevalence is high in this group of homosexual men, but many infections remain undiagnosed.

"High levels of unprotected anal intercourse continue to be reported by both HIV saliva antibody positive and negative men and the potential for onward transmission of HIV and increasing prevalence is a major public health concern."

Recently, a witness in a court case was informed that he had the virus while giving evidence, much to his distress.

He had been tested without his knowledge after giving a sample to police.

The judge ordered an inquiry into the incident.

Earlier this year the Health Protection Agency (HPA) reported a 20 per cent increase in people being diagnosed with HIV during 2003.

The HPA revealed in February that 5,047 new cases had been reported so far last year - compared to 4,204 for the same time the previous year.

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 epidemic.