Two leading sexual health charities are calling for children as young as four to be given compulsory sex education.
Brook and the Family Planning Association (FPA) argue that teaching children about sex from a young age would help reduce abortion rates and sexually transmitted infections when they reach adolescence.
The charities said children should be taught the names of body parts and about sex and relationships.
The Brook chief executive, Simon Blake, is calling for all UK governments to ensure that all children age four upwards receive sex and relationships education in schools.
Blake said: "If we get high quality sex and relationships education in every primary and secondary school across the UK, all the evidence shows teenage pregnancy rates will continue to fall and will improve young people's sexual health.
"Whilst sex and relationships education continues to be patchy, another generation of children and young people do not get the education they need to form healthy relationships and protect their sexual health."
Brook wants every primary and secondary school to be legally required to provide sex and relationships education and secondary schools to ensure that young people have access to free confidential contraceptive and sexual health services.
He told the BBC: "Many young people are having sex because they want to find out what it is, because they were drunk or because their mates were.
"That's just not good enough for young people. We've got to have high expectations for them so they've got high expectations for themselves."
He added: "All the evidence shows that if you start sex and relationships education early - before children start puberty, before they feel sexual attraction - they start having sex later.
"They are much more likely to use contraception and practise safe sex."
The Department for Children, Schools and Families issued new draft guidance on wellbeing for schools yesterday.
The Sex Education Forum (SEF), the national authority on sex and relationships education, called for Personal, Social, Health and Economic education (PSHE), which includes sex and relationships education, to be made statutory.
Anna Martinez, head of the SEF, said: "Considering the value placed on PSHE in the guidance, SEF is once again urging government to consider making it a statutory subject, to give PSHE the high status it deserves as an essential part of all children's education."
Julie Bentley, chief executive of the FPA, said: "This is not about teaching four-year-olds how to have sex but teaching them about respecting themselves and others and giving them the confidence to ask questions when they get older and do want to find out more.
"It's like maths - at primary school children learn the basics so that they can understand more and more complex concepts at a later stage."
She added: "Parents are concerned that if they are told about sex they will go straight out and have it but the research shows the complete opposite. They have sex later and when they do they have safer sex."
At present, all children have to learn about the biology of reproduction but parents can opt to remove children from PSHE lessons, where they learn about the emotional and relationships side of sex.