The Mayor of London Boris Johnson said: "London is home to almost half of all people living with HIV in the UK, but a quarter of them are unaware they carry the virus.
"It is vital that people who might be at risk get tested, not only to reduce the risk of transmission to others, but to ensure they get the life-saving treatments that are available."
More top news
Ex-London mayor Ken Livingstone says he is sorry for the "disruption" caused by his comments that Adolf Hitler was a supporter of Zionism.
Kickstarter group asks for public donations to fund replacing Tube ads with pictures of cats
Sir David Attenborough opens Woodberry wetlands in Stoke Newington