A fifth of adults do not know HIV can be transmitted between people not using a condom, according to the National Aids Trust.
Around half do not know that sharing needles when injecting drugs can also pass on the virus.
The trust said more adults (now a rate of around 10%, compared with 5% in 2007) believe HIV can be passed by kissing and spitting.
The group conducted a survey among 2,000 people, which is included in an HIV public attitudes report. They were given a list of 11 possible routes of HIV transmission and were asked to select those which applied to the disease.
Only 30% of people could correctly identify all HIV transmission routes, while one in 12 adults (8%) could not identify any sex without a condom as a route.
Deborah Jack, Chief Executive of the trust, said: "As the number of people with HIV in the UK approaches 100,000, it is crucial for everyone to understand the facts around how HIV is passed on, so they can protect themselves and others.
"Whilst HIV disproportionately affects gay men and Africans, the number of people with HIV who are not in these groups is steadily rising, and unfortunately there does still remain a serious amount of confusion around HIV transmission."