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
A poll of Britain's largest trade union, seen by ITV News, indicates more than half of those surveyed think the Labour leader should quit.
Murder investigation launched in Belsize Park after a woman dies in hospital from injuries.
Police launch a manhunt for an escaped violent sex offender after a boy was attacked and woman had her hair 'sprayed red' in west London.