Children as young as seven in Britian will be introduced to puberty and sexual intercourse in compulsory sex education lessons, a new official guidance reveals.
The new guidelines said pupils will be taught about how boys and girls develop, including the production of sperm and eggs, before they leave primary school.
The lessons for students from 7 to 11 will also focus on the importance of strong relationships and marriage, as well as the risks of abuse and domestic violence in the home.
In the first three years of secondary education, children will be encouraged to talk about the "different ways of expressing sexual intimacy".
Contraception, sexually transmitted diseases and abortion will also be covered.
The move comes despite fears from family groups that children are being exposed to detailed lessons on sex before they are ready.
Campaigners said that existing classes had failed to stop teenagers getting pregnant in record numbers. Britain already has the highest teenage pregnancy rate in western Europe.
But new guidelines from the Department for Children, Schools and Families said sensitive issues were often addressed "too late" and exploring the subject at a young age did not "encourage them to be sexually active".
Sex education will be covered as part of mandatory personal, social, health and economic education (PSHE).
"Young people today grow up in a very different world to the one their parents knew as children," the Telegraph quoted Ed Balls, the Children's Secretary, as saying.
"New technologies and a 24 hour media mean that young people are increasingly exposed to images and content that can make them feel pressure to be sexually active before they are ready and can give them misleading information about relationships and growing up.
"We want to give young people the facts so that they can stay safe and healthy. We also want young people to understand the importance of marriage and other stable relationships - these are the bedrock of family life, the best way to bring up children and the kind of relationships we want young people to develop as they get older," he said.
But Norman Wells, from the Family Education Trust, said the new guidelines will deprive children of their moral scope.
"This draft guidance will confirm the fears of many parents that compulsory sex education will be used to indoctrinate their children into thinking that there are no moral absolutes when it comes to sexual expression," he said.
"In the name of non-judgmentalism, the Government's approach is abandoning young people to the shifting sands of relativism and depriving them of the moral compass they so desperately need.Contrary to the Government's claims, introducing sex education at an early age runs the risk of breaking down children's natural sense of reserve.
"Far from being a hindrance, children's natural inhibitions and sense of modesty in talking about sexual matters are healthy and provide a necessary safeguard against both sexual abuse and casual attitudes towards sexual intimacy later on," he stated.