More than a quarter of 11-year-olds in the UK know someone their age who is having sex, according to a new survey.
The charity, Marie Stopes International, commissioned the study of 1,077 children aged from 11-15.
It intends to use the findings to win support for a new campaign it is running to supply any child logging onto its website with free condoms on request, which will be posted to them at home.
More than half of those surveyed did not know that condoms could prevent HIV while one in 10 believed the virus could be passed on through kissing.
The study also revealed that more young people were concerned about contracting a sexually transmitted infection or getting pregnant than they were about doing well at school.
Sam Guy, Marie Stopes International's Adolescent Advisor, said:, "Not only are young people unaware of the benefits of condoms, many are also too embarrassed to actually buy and use them.
We know from the thousands of letters we get every day at Sugar that the message about safe sex is just not getting through
Girls' magazine editor
"Marie Stopes International UK hopes to overcome these factors by encouraging young people to find out about condoms and become comfortable with them before they think about having sex."
He said one particularly worrying statistic was that even where young people had received sex education at school, their levels of awareness were not significantly higher.
Despite 77% saying they had some form of sex education at school, more than a quarter said they needed further information.
The survey results are being published in Sugar, a magazine for teenage girls.
Editor Claire Irvin said the survey proved children were still confused about sex and contraception.
"We know from the thousands of letters we get every day at Sugar that the message about safe sex is just not getting through," she said.
"Shocking results such as these help underline Sugar's message about sexual health and we will continue to give our readers the advice they need on safe sex when they are in a loving and trusting relationship and ready to have sex."
The charity said the purpose of the research was to investigate children's experiences of sex education and knowledge of sex, contraception and related issues.