More than 20,000 people with HIV are unaware they are carrying the virus and are infecting thousands of others, setting a devastating health “timebomb”, medical experts have warned.

Senior doctors have accused the Department of Health of failing to take the spread of HIV seriously and neglecting to test enough people in high-risk groups, including gay men and heterosexual black Africans.

HIV specialists say they are seeing people in clinics with full-blown Aids who have no idea they have been carrying the virus. They now want all sexually active people to be routinely offered an HIV test.

The Lancet medical journal has published an editorial accusing ministers of an “appalling failure to tackle HIV” and of having “no credible strategy to diagnose and care for those living with, but unaware of, HIV in Britain”.

The Health Protection Agency (HPA), the government’s health watchdog, warned that about 77,000 people in the UK have HIV but 21,000 of these do not know they are infected. In 2007, the number of infections through heterosexual contact increased to 960, up from 540 in 2003.

Doctors warn that a third of people with HIV are being diagnosed when their virus is advanced. One London hospital recently treated two teenage sisters, one of whom was pregnant, infected with HIV from the same man. Hospitals are also concerned about men who are diagnosed with HIV but abscond before they can be treated.

They called on the government to take testing more seriously, warning that an A&E target to treat patients within four hours meant people with early symptoms of HIV were not being tested in emergency rooms because of time pressure.

Dr Phillip Hay, reader in HIV medicine at St George’s hospital in Tooting, south London, said testing for the virus should be routine to stop its spread through unprotected sex.

He said: “We have identified some people who have infected multiple individuals”, including couples “where there is a big difference in age between an older adult and a teenager. All men and women accessing medical care should be routinely offered a test”.

The HPA said high-risk groups should be targeted for testing.

“It is a matter of concern that so many individuals in the UK are unaware that they are HIV-infected,” it said.

The Department of Health said HIV prevention was still a priority.

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