Too many people are not educated about HIV and Aids, the Prime Minister has said, after figures suggest that a record number of people in the UK are living with HIV.
David Cameron said that almost 25,000 people in the UK who have the human immunodeficiency virus are not aware they have been infected.
These people are not benefiting from treatment and are increasing the risk of passing the virus on, he said.
Mr Cameron said that World Aids Day should be used to "spread the message" about education, testing and early diagnosis.
The international annual campaign, symbolised with a red ribbon, is about increasing awareness of the disease, celebrating breakthroughs in treatment and remembering those who have died as a result.
"World Aids Day is a chance to do two things: take action and reflect," said Mr Cameron.
"First and foremost, it's got to be a spur to take action, as there are still far too many people who aren't educated about HIV and Aids and who don't get tested.
"Over 100,000 people in the UK are living with HIV, but a quarter do not know they have it - that is 25,000 people who are not benefiting from treatment and are increasing the risk of passing the virus on.
"This means up to 600 people a year could be dying from a preventable, treatable condition.
"People need to know that, diagnosed early, the outlook for most people with HIV in the UK today is a good one, thanks to the availability of effective treatment and the excellent care provided by the NHS.
"Above all, people need to know that it's never been quicker or simpler to get tested."
Latest data from the Health Protection Agency shows that the number of people living with the virus in the UK has almost hit 100,000.
There were 6,280 people diagnosed with HIV last year, the HPA said.
New diagnoses among gay men reached an all-time high in 2011, with 3,010 men discovering they were infected.
The HPA said that one in 20 men who have sex with men in the UK now have HIV, and the figure soars to nearly one in 12 in London.
Black African people are also at a higher risk, the HPA said, with 37 per 1,000 living with the infection - overall HIV prevalence in the UK was 1.5 per 1,000 people.
HIV attacks the immune system and weakens the ability to fight infections and disease.
Aids is the final stage of HIV infection, when the body can no longer fight life-threatening infections.
HIV is found in the body fluids of an infected person, and about 95% of people are infected through sexual contact.
Mr Cameron reaffirmed his pledge to spread the message by taking to Twitter this morning.
In a tweet to his 170,000 followers, the Prime Minister wrote: "Today is #WorldAIDSDay - a time to raise awareness, challenge stigma & pledge support for our hardworking AIDS charities."
Labour leader Ed Miliband also backed the campaign with a message on his official profile. It read: "World Aids Day is an incredibly important moment when we recognise the need to continue the fight against HIV and Aids."