Around one in five people living with HIV in the UK are unaware they have the virus, figures show.
Some 21,900 people out of 98,400 with HIV do not know they have it, according to 2012 data from Public Health England, although the figures are slightly down on the previous year.
Some 47% of the 6,360 people newly-diagnosed with HIV in 2012 were diagnosed at a late stage while the overall number of new cases was up 1% on the previous year.
New HIV diagnoses among men who have sex with men reached an all-time high, with 3,250 cases in 2012.
Some 45% of new cases of HIV were among heterosexuals, accounting for 2,880 cases.
Professor Noel Gill, head of PHE's HIV department, said: "In the UK, people who are unaware of their infection are likely to be those most at risk of transmitting HIV to others.
"We must increase the speed at which we're reducing the number of undiagnosed HIV infections by encouraging earlier and more frequent HIV testing, especially by those most at-risk.
"Earlier diagnosis will help reduce new HIV infections across the UK.
"Around half of men who have sex with men recently diagnosed with HIV received their diagnosis the first time they tested, which is a strong indication that many men who should be testing are not."
Paul Ward, acting chief executive of the Terrence Higgins Trust, said: "Reducing undiagnosed HIV is now the single most important step we can take to halt the spread of infection in this country.
"Some communities are already making headway in this.
"Among gay men, testing rates are up, diagnoses are up, and as a result undiagnosed infection is coming down.
"Because of community-wide initiatives like National HIV Testing Week, hundreds more people with HIV now know their status, helping them access life-saving treatments and drastically reducing the chance of them passing the virus on.
"We've come so far, but we have to keep going.
"We've never been in a stronger position to beat the virus, with cutting-edge testing services and free, world-class drug treatments for anyone who tests positive.
"We know testing works and treatment works; all we need is the individual commitment and public funding to make it happen.
"If we can get this, we can turn the tide of the epidemic."
National HIV Testing Week runs from November 22 to 29.