A record number of people in the UK are living with HIV, with almost one in 12 gay and bisexual men in London affected, new figures show.
The Health Protection Agency (HPA) said there are about 96,000 people who have the virus in the UK - an all-time high.
But health officials warned a quarter of people who have the human immunodeficiency virus are not aware they have been infected.
There were 6,280 people diagnosed with HIV in 2011, the HPA said.
Nearly half of all diagnoses in 2011 were acquired heterosexually. Of these, more than half were probably acquired in the UK, compared to only 27% in 2002 according to the HPA report.
New diagnoses among gay men reached an all-time high in 2011, with 3,010 men discovering they were infected.
The HPA said that one in 20 men who have sex with men in the UK now have HIV, while the figure soars to nearly one in 12 in London.
Black African people are also at a higher risk, the HPA said, with 37 per 1,000 living with the infection - overall HIV prevalence in the UK was 1.5 per 1,000 people.
Dr Valerie Delpech, HPA head of HIV surveillance, said: "These figures are a reminder of how vital safe sex programmes remain. Promoting HIV testing and condom use is crucial to tackling the high rates of transmission, late diagnosis and undiagnosed HIV still seen in the UK.
"National HIV Testing Week is a great opportunity to encourage people to get tested. We also encourage clinicians to take every opportunity to offer the test to those in higher risk groups and, in high prevalence areas, to all general medical admission and new GP registrants."