Teenagers lack knowledge about condom use, the study suggests
Nearly half of all under-25s do not use condoms with new partners, according to a new survey.
The report by the National Aids Trust also found that more than 200 young people questioned rarely or never used a condom.
The finding has prompted the body to call on the government to introduce condom advertising on TV and radio before the nine o'clock watershed.
The Trust is also lobbying for condom awareness to form an essential part of compulsory education in all schools.
The call comes during National Condom Week.
Deborah Jack, chief executive of the National Aids Trust, said: "In National Condom Week, the British public need a wake-up call on condom use.
"Too many people don't know the basic facts on how condoms prevent a sexually transmitted infection such as HIV.
"Too many people are inconsistent in how they use condoms and too many people give up on condoms in a relationship without a health check-up, thus possibly harming their health or that of their sexual partner.
"We need to educate the whole population in the importance of condoms."
The Ipsos MORI survey suggests that 49% of Britons don't always use a condom with a new sexual partner.
Some 24% of people questioned who had a new sexual partner within the last two years said they would only stop using a condom once they had both been tested for HIV and other sexually-transmitted infections (STIs).
A further 17% said they would always use a condom.
That leaves over half - 53% - putting themselves at risk of an STI when they stop using condoms in a relationship.
Doctors say they're seeing more problems in their surgeries because of this.
GP Anhoushka Hari told Newsbeat she's treating more and more people with STIs and has also seen an increase in accidental pregnancies.
"I'm seeing more Chlamydia, Gonorrhoea and infections like HIV and Hepatitis," she says.
"A lot of these diseases are silent, so you can be carrying and transmitting one without having any signs of it at all."
National Condom Week is taking place until 18 May.