They say that three out of four pregnant women in London found to be HIV positive through anonymous screening carried out in a national programme authorised by the Department of Health will not know that they are infected.
Similarly, among heterosexuals attending clinics for sexually transmitted diseases across the country the researchers estimate that 60 per cent of men and 53 per cent of women will be unaware of their HIV status - as will be 14 per cent of injecting drug addicts attending drug centres.
Even among homosexual and bisexual men, the group best educated about the risks and safer sex practices, the estimate for those who are HIV positive but do not know is 29 per cent.
In the latest Communicable Disease Report, the scientists warn health workers to be on their guard and to 'appreciate the extent of under-diagnosis in various clinical settings as these infected persons may not have had the opportunity to benefit from counselling and clinical monitoring'.
The estimates have been reached using statistical checks including Aids forecasting, back calculations and comparing known HIV cases from those who have volunteered to be tested with the prevalence of HIV found by the anonymous mass screening.
The scientists estimate that 45 out of 61 pregnant women with HIV will not know they were infected. Among heterosexuals attending clinics for sexually transmitted diseases, only 27 out of 67 men and 24 out of 51 women will be aware of their HIV status. Among homosexual or bisexual men, the estimate is that 549 out of 777 HIV cases will know.
Dr Noel Gill, chairman of the Public Health Laboratory Service Aids Centre, said that the latest figures reinforced the need for the controversial 'unlinked, anonymous' screening.
Between January 1990 and July 1992, blood samples from nearly 500,000 men, women and babies were screened for HIV in a variety of settings: ante-natal departments and blood spots from the new born; clinics for sexual diseases; centres for injecting drug users; and a much smaller number of general hospital patients.
The aim is to discover the true incidence of HIV infection in the community. The samples are unlinked and anonymous so they cannot be traced back to the individuals, who are not told of the results. But patients are told that their blood sample will be tested for HIV and they are offered a voluntary test and counselling.
Dr Gill said new Department of Health guidelines to health authorities already reflected their findings on the undiagnosed pregnant women. Authorities have been told to assess the local risk and encourage pregnant women attending ante-natal clinics to take the HIV test voluntarily.
'Knowing you are HIV positive does not necessarily mean you change your behaviour. The number of gay men, with HIV, for example who are newly diagnosed with rectal gonorrhoea is causing us concern.'
These cases indicate that the men are having unprotected sex with new partners.