More than 20 years since the first UK advertising campaign against HIV urged people "Don't die of ignorance" the number contracting the disease in the UK has almost doubled in a decade.

New diagnoses of people infected with HIV rose from 1,950 in 2001 to 3,780 in 2010, according to the Health Protection Agency (HPA). A key driver of the rise is that one in four of those infected does not know they are HIV positive.

An expansion of HIV testing in the 38 Primary Care Trusts with the highest HIV prevalence is recommended in new guidance from the National Institute for Clinical Excellence (NICE).

Most of the PCTs – 27 – are in London. The remainder are in major towns on the south coast (including Brighton and Bournemouth) and in the Midlands and North (including Birmingham, Nottingham and Manchester).

Routine testing for HIV should be offered to all patients, regardless of age or ethnic background, who are registering with a GP, taking a blood test or being admitted to hospital in these areas, NICE says. The aim is to avoid stigmatising the groups with the highest incidence of the disease – gay men and black Africans.

The lifetime cost of treating someone with HIV is almost £320,000 and the HPA says the NHS could have saved £1.2 billion if all 3,780 cases infected in the UK in 2010 had been prevented.

Many patients are diagnosed late, harming their own health – the virus damages the immune system – as well as putting others at risk. On average, of all those who die from HIV infection every year, three out of five are diagnosed late. Men who have sex with men are at highest risk of contracting the disease in the UK, with new diagnoses up 70 per cent in the last 10 years.

Fear of HIV in the late 1980s and 1990s curbed sexual activity among heterosexual and gay young people. But complacency following the arrival of anti-retroviral drugs in the mid-1990s, which fostered the perception that HIV was a treatable disease.

Valerie Delpech, head of HIV surveillance at the HPA, said: "HIV is an extremely serious infection. There are excellent treatment options available nowadays but these are only at their most effective if the infection is diagnosed early, before symptoms appear. Testing must be increased to catch the infection as early as possible."

Professor Mike Kelly, director of the Centre for Public Health Excellence at NICE, said: "For many people of black African heritage there is a fear that being diagnosed HIV positive will result in social exclusion or racism... from both inside and outside their community. There is often a reluctance to be tested which can delay diagnosis." He said the NICE guidance aimed to "normalise HIV testing" by making it routine in areas of high HIV incidence.