More than a quarter of people with HIV are undiagnosed, although the overall number of new cases is falling, new figures have suggested.
An estimated 86,500 people were living with HIV in 2009, but more than a quarter of those - almost 22,500 - are unaware they have the infection.
The data, published by the Health Protection Agency (HPA), revealed there were 6,630 new cases of HIV in 2009 among 4,400 men and 2,230 women.
This is the fourth year-on-year decline in new cases although the proportion of undiagnosed cases has not changed in the past decade.
Across the UK, 1.4 people per 1,000 of the population have HIV, whether diagnosed or undiagnosed. And 52% (3,450) of people newly diagnosed in 2009 were diagnosed late - after a time when treatment could have started.
The HPA now wants to see more widespread testing, especially in areas where the number of new cases is above two people per 1,000 population.
Of 37 health trust regions across England with levels above this, 26 are in London. Across the capital, 5.24 per 1,000 people had a diagnosed HIV infection in 2009.
Brighton and Hove has a rate of 7.57 per 1,000 people and Manchester has a rate of 5.22 per 1,000 people.
People who are most at risk include gay men and black Africans, health officials said.
Dr Valerie Delpech, consultant epidemiologist and head of HIV surveillance at the HPA, said: "We're very concerned that a large number of people in the UK are unaware of their HIV status and that half of all newly diagnosed people are diagnosed late, meaning they may not benefit from very effective treatments."