A charity claims the government will miss its target to reduce the number of gay men infected with HIV leaving sexual health clinics undiagnosed.
The National Aids Trust (NAT) is now pushing for the introduction of an "opt out" scheme for HIV testing in UK clinics.
Currently, anyone attending a site for screening has to be specifically asked if they want to be tested for HIV.
But the NAT says it should be automatically included in the test unless patients choose to opt out of having an assessment.
Statistics from the Health Protection Agency show that the number of gay men with HIV who remain undiagnosed after leaving a clinic stood at 55% in 2001. It then promised to cut this figure by 50% by the end of 2007.
But it had only dropped to 43% by 2005, and the charity claims the government is unlikely to get this down to 27.5% by the end of this year.
NAT chief executive Deborah Jack said: "It is vital that people living with HIV are diagnosed as early as possible, both for the sake of their own health and to prevent the virus being passed on to others."
A Department of Health spokesman said: "HIV testing is already done as part of a routine sexual health screen in some clinics, but it is important that where HIV testing is done on an 'opt-out' basis, that individuals give consent."