A quarter of people with HIV in the UK, over 22,000, are unaware they have the virus, experts from the Health Protection Agency have suggested.
The 2009 estimate is based on anonymous blood samples supplied for other reasons but tested and found to be HIV positive.
It is at the same level as 2008's estimate.
The Terrence Higgins Trust said more investment was still needed in prevention and awareness programmes.
The Health Protection Agency said 6,630 people were newly diagnosed with HIV in 2009, down for the fourth year in a row.
The HPA's report also found over half of those were diagnosed after the stage when treatment should have started.
In all, the number of people living with HIV in the UK reached an estimated 86,500 last year.
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."
She called for better access to HIV testing in areas where rates of HIV infection are high.
"Pilot studies have shown that in these areas testing all adults registering at GPs or accessing certain hospital services can make an impact."
"The evidence shows that this testing is feasible to undertake and acceptable to patients. We would like to see this rolled out in areas where HIV infection is more common to reduce the number of people who are unaware of their HIV status and increase the chances of early diagnosis, when treatment is more successful."
The HPA added that those at higher risk of HIV, such as men who have sex with men and black Africans living in this country, should know their status and consider being tested regularly.
Sir Nick Partridge, chief executive of the Terrence Higgins Trust sexual health charity, said: "With more people living with HIV than ever before in the UK it's vitally important we invest in targeted HIV prevention and awareness programmes, particularly for gay men who are most at risk.
"We also need to re-double work to get people to test, to reduce the high numbers of people who don't know that they have HIV for far too long before being tested and prevent needless deaths."