Too many people are refusing HIV tests at sex health clinics - and a key government target could be missed as a result, says an Aids charity.
A blood test can confirm HIV
The target aimed to halve the number of people whose HIV infection is missed when they visit a sexual health clinic for another reason.
The National Aids Trust said that making an HIV test automatic could solve the problem.
The Department of Health said it was reviewing policy on testing.
Many people who have contracted HIV remain unaware of this, as it may be some time before symptoms begin to appear - it is estimated that a third of HIV-positive men and women in the UK don't know they have the virus.
However, during this period, they may be able to pass the virus on to others through unprotected sex or sharing drug needles.
The government's National Strategy for Sexual Health pledged, by the end of this year, to cut by 50% the proportion of people infected with HIV who remain unaware of their infection even after a visit to a sexual health clinic for another reason.
Experts can work this out because the Health Protection Agency tests random, anonymous blood samples, even where the patient has refused an HIV test, providing a figure for the underlying rate of infection among people visiting clinics.
In 2001, 55% of gay men with undiagnosed HIV visited a sexual health clinic, and left without a diagnosis.
In the latest figures taken in 2005, this had fallen to 43%, well short of the target of 27.5%.
The target is likely to be met among heterosexual patients, with the figure falling from 48% in 2001 to 27% by the end of 2005.
At the moment while people are offered and encouraged to take an HIV test, unless they actively "opt in", the test won't take place.
The National Aids Trust now wants an "opt-out" system for HIV testing, with the presumption that the test will go ahead unless the patient actively refuses.
Deborah Jack, Chief Executive, 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.
"It is estimated that there are 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 help reduce those numbers, particularly among gay men."
Nick Partridge, Chief Executive of the Terrence Higgins Trust supported the call: "Routine HIV testing in sexual health clinics is both appropriate, cost effective, and long overdue.
A spokesman for the Department of Health said that there had been "encouraging progress" among heterosexual men and women visiting clinics.
He said: "The Department is currently working with the Expert Advisory Group on Aids and others including the Terrence Higgins Trust, to review our policy on HIV testing in sexual health clinics.
He added that it was important that, even if an opt-out system was introduced, that proper consent was obtained from patients.