More than 8,000 people in London are unaware they have HIV, according to figures in a report.
The report reveals that HIV is rising fastest among heterosexuals
The London Health Observatory (LHO) said there were about 8,600 undiagnosed cases of HIV, which is rising fastest among heterosexuals.
It said nearly all new infections were among black Africans, most of whom had acquired the virus abroad.
A three-fold rise in black Caribbeans receiving treatment for HIV is recorded among 20,000 Londoners who have it.
Bobbie Jacobsen, of the LHO, said: "What we've shown is that there are just over 20,000 diagnosed cases. We've estimated from research done already that just under 30% are likely to be undiagnosed.
"There are now over 8,500 young people walking around in London who don't know that they are HIV positive."
The report said the largest group of people receiving treatment for HIV in London was homosexuals.