A charity is calling for GP clinics to introduce "opt out" HIV screening as a routine part of sexual health check ups.
Currently patients have to ask for a HIV test, but the National AIDS Trust says this means many with HIV infections go undiagnosed.
Infection among gay men is particularly high, and 43% attending sexual health clinics are not diagnosed.
The government's National Strategy for Sexual Health and HIV aims to reduce HIV infections by 50% in those who visit a sexual health clinic by the end of 2007.
Chief Executive of the National AIDS Trust Deborah Jack commented: "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 onto others.
"It is estimated that there are over 20,000 people with HIV in the UK who have not yet been diagnosed.
"Making an HIV test a routine part of a sexual health check up could really reduce these numbers, particularly among gay men."
"I have advocated routine HIV tests for some time. If it were done as a routine test, eventually the stigma disappears, as has happened with the other STD tests that are now part of routine ante-natal care" - Name and address supplied