An estimated 86,500 people are living with HIV in the UK, according to figures released today, but around a quarter of them do not know they are infected.

"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," said Dr Valerie Delpech, head of HIV surveillance at the Health Protection Agency which published the figures.

They show that HIV is more prevalent in certain areas of the country – particularly in London, Brighton and Manchester. Pilot projects looking at the acceptability of increased testing in such areas are underway.

"The HPA would like to see increased access to … 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," said Delpech.

"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."

People who seek medical help only when they have the symptoms of Aids are far more likely to die. In 2009, 547 people were diagnosed with Aids and hardly any of them had been for an HIV test. Of the 516 people with HIV infection who died last year, 73% had been diagnosed late, when the virus had already taken a big toll on their immune system.

In 2009, 6,630 people were newly diagnosed with HIV, 4,400 of whom were men and 2,230 women. This is a decline in new diagnoses, for the fourth year in a row, but the agency says there is no evidence of a drop in the numbers who are living in ignorance of their HIV status.

Deborah Jack, chief executive of the National Aids Trust, said the figures showed the need to increase testing. "Prevention is an immensely cost-effective activity … one HIV transmission is up to £360,000 in direct costs to the NHS."

The agency warned in July that the number of new HIV infections among over-50s had more than doubled in seven years.