One in three people who discovered they had HIV last year were diagnosed late and could not benefit from early treatment, figures suggested today.
Data from the the Health Protection Agency showed there were 7,298 new diagnoses of HIV among over-15s in 2008.
Of these, an estimated 32 per cent (2,310 people) were diagnosed at a point after which treatment should already have begun - at a CD4 cell count less than 200 per cubic millimetre of blood.
If new guidelines from the British HIV Association, published last year, are applied, 55 per cent of people were diagnosed late.
The guidelines say patients should get treatment when their CD4 cell count is less than 350 per per cubic millimetre of blood, rather than waiting for it to fall under 200.
A total of 525 people died from HIV in 2008, comparable with previous years, the HPA data also showed.
However, three-quarters of these were diagnosed late.
There are complex reasons behind late diagnosis, including people putting off going for testing and tests not always being available in the right settings, according to the HPA.
Due to better therapies, the number of people living with HIV is increasing, to around 83,000 in 2008.
A total of 100,000 more tests were carried out at genitourinary medicine clinics (GUM) in 2008 than in 2007, with some 950,000 being performed, up from 850,000 in 2007.
More than nine out of 10 (93 per cent) people attending GUM clinics are now tested for HIV, up from 77 per cent in 2004.
Health trusts in those parts of England with higher than average rates of HIV are encouraged to routinely offer testing to everyone aged 15 to 59.
The idea is to test people when they register with a GP or when they are admitted to hospital.
Dr Valerie Delpech, an HIV expert for the HPA, said: "HIV is a serious infection but if diagnosed early, there are very good treatment options.
"Of concern is that over 22,000 people remain unaware of their infection in the UK and cannot therefore benefit from effective treatment.
"It is encouraging to see the increased numbers of HIV tests being performed in STI clinics.
"However, we must reduce the number of people presenting late with their infection by increasing awareness of the importance of testing, improving access and de-stigmatising the HIV test.
"We need to continually reinforce the safe sex message - using a condom with all new or casual sexual partners is the surest way to ensure you do not become infected with a serious sexually transmitted infection such as HIV."
Sir Nick Partridge, chief executive of the Terrence Higgins Trust, said: "The level of undiagnosed HIV in the country is completely unacceptable.
"With early diagnosis and effective treatment, most people with HIV can live to old age.
"If left undiagnosed, they will die earlier, be significantly more ill and more likely to infect others.
"HIV testing is easy, quick and saves lives. There should be more testing in more settings, and we need the political will to make this happen."
The charity is calling on the Government to introduce a national, targeted, screening programme with the aim of cutting undiagnosed HIV cases by 50 per cent by 2014.