The number of people living with HIV in the UK reached an estimated 91,500 in 2010, with a quarter of those unaware of their infection, according to official figures published on Tuesday.
The figures came from the Health Protection Agency (HPA) before the World AIDS Day on December 1.
The HPA has called for universal testing for HIV, so that no one leaves an STI (sexually transmitted infections) clinic without knowing their HIV status.
The HPA is concerned that over half of people diagnosed in 2010 came forward for testing after the point at which treatment for their infection should ideally have begun.
Late diagnosis is associated with an increased risk of AIDS and death.
The HPA's annual 'HIV in the UK' report found 6,660 people were newly diagnosed with HIV in the UK.
The report confirmed that infections probably acquired within the UK almost doubled in the last decade from 1,950 in 2001 to 3,640 in 2010 and exceed those acquired abroad -- 3,020.
This rise is mostly due to infections acquired among men who have sex with men, who remain the group most at risk of HIV infection in the UK.
Dr Valerie Delpech, consultant epidemiologist and head of HIV surveillance at the HPA, said: "HIV is an infection which can nowadays be treated and those diagnosed promptly can expect to experience similar life expectancy as an individual without the infection.
"However, we are very concerned that a large number of people in the UK are unaware of their HIV status and are diagnosed late".