Annie Lennox said in an interview with London Tonight: "Things have changed radically over the last three decades. So much has been done scientifically to find effective medical treatment that can help to save lives. But we're still struggling with the issue of stigma, fear and ignorance.
"There are many people that need to get tested. We're going to be encouraging people to go and get tested and find out their HIV status. Because HIV is very often a sexually transmitted thing, there's that stigma, there's that fear.
"Sex is something people are not comfortable talking about. It's very important to bring the dialogue out into the open...the sooner you get tested the more effective the treatment will be."
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."
The Terrence Higgins Trust is the UK’s main HIV and sexual health charity, providing a huge range of services to over 100,000 people annually.
It also campaigns and lobbies for greater political and public understanding of the personal, social and medical impact of HIV and sexual ill health.
Terry Higgins was one of the first people in the UK to die with AIDS. He died aged 37 on 4 July 1982 in St Thomas' Hospital, London. By naming the Trust after Terry, the founder members hoped to personalise and humanise AIDS in a very public way.
The charity was set up in 1982 by a friend of Terry’s, Martyn Butler, and Terry’s partner, Rupert Whitaker, with the aim of preventing others from having to suffer as Terry did.
It focused on raising cash for research and awareness of the illness that was then called Gay-Related Immune Deficiency.
The Mayor of London, his HIV Ambassador pop star Annie Lennox and the Terrence Higgins Trust are celebrating 30 years of the charity and the HIV response in London.