"Unsafe sexual behaviour" and a lack of testing is being blamed for a failure to cut the number of cases of HIV among gay and bisexual men in the last decade.
The number of cases of the virus in England and Wales among men who have sex with men (MSM) has stayed the same over 10 years despite a significant increase in availability of testing and treatment, according to a new study.
One in 20 gay and bisexual men in the UK now has HIV, rising to one in 12 in London, according to the Medical Research Council (MRC) and Health Protection Agency (HPA), which carried out the research.
Dr Valerie Delpech, the HPA's head of HIV surveillance, said: "We are seriously concerned about the level of ongoing HIV transmission and the significant impact this is having within the MSM community.
"The most plausible explanation for these results is continuing unsafe sexual behaviour coupled with insufficient HIV testing, showing us just how vital safe sex programmes engaging MSM remain."
The study, published in the Lancet Infectious Diseases journal, found no evidence of a decline in levels of HIV over the last decade, with new infections plateauing at 2,300-2,500 annually from 2006 onwards. Undiagnosed HIV infections in gay and bisexual men increased from 7,370 in 2001 to 7,690 in 2010.
Despite a 20% reduction in the average time between infection and diagnosis, from four years to 3.2 years, 38% of infections in 2010 were still diagnosed after the time patients should have started antiretroviral (ARV) treatment.
Over the same time period HIV testing of gay men and bisexual men increased by 370% to 59,300 per year and the number receiving HIV care rose from 69% to 80%.
A Department of Health spokesman said: "HIV continues to be a serious health issue, especially for gay and bisexual men, and people from African communities.
"We urge people at risk to get tested. Free and confidential HIV testing is widely available from the NHS. We will set out detailed plans for sexual health and HIV services shortly."