Sex education is to be made a compulsory part of the national curriculum in primary and secondary schools under government plans to cut teen pregnancies and sexually transmitted diseases.
A new personal, social and health education (PSHE) curriculum, expected by 2010, will include compulsory sex and relationships education as well as better advice warning children against drugs and alcohol.
Children will learn about body parts and the fact that animals reproduce from the age of five, puberty and intercourse from the age of seven and contraception and abortion from the age of 11.
Schools will not be allowed to opt out of the rules but the government is promising separate guidance to faith schools, which could find elements of the new curriculum at odds with their spiritual beliefs.
The schools minister, Jim Knight, said they would still have to teach the curriculum - which includes contraception, abortion and homosexuality - but will separately be allowed to continue to teach religious beliefs about sex.
Knight said he wanted all schools to teach children more about sex in the context of relationships, including marriage and civil partnerships, and to promote abstinence.
"We are not talking about five-year-old kids being taught sex. What we're talking about for key stage 1 is children knowing about themselves, their differences, their friendships and how to manage their feelings," he said.
Secondary schools have so far had to teach the mechanics of sex in biology classes, but not in conjunction with relationships and sexual health. The new lessons will be part of wider lifestyle classes that will include drugs and alcohol.
Presenting the findings of a government review, Knight said pupils will also be taught about:
• The laws against drug and alcohol misuse and the risks involved
• Healthy lifestyles, the need for a good diet, and exercise
• How to manage their money
The government said it wanted parents to be informed about the contents of sex education classes and set out plans to improve the teaching of classes, using dedicated teachers and training.
Knight said that a review led by Sir Alasdair MacDonald, an east London headteacher, would investigate further how compulsory PSHE education would be introduced and consider whether parents or even schools should be given an opt-out.
But Knight said: "We wouldn't be suggesting a statutory programme of study if we thought schools would have an opt-out.
"There are some that argue having an opt-out for parents for the national curriculum is difficult, but I think it is important that individual parents views are taken into account and their right to withdraw because of personal or moral views is respected. It's something it would take a lot for us to move away from."
He said that supplementary guidance would be produced for Catholic schools advising them that they too must teach all elements of the curriculum, but they will be allowed to continue to teach the Catholic values about contraception, abortion and homosexuality.
"I'm absolutely fine with Catholic schools educating their children in accordance with their faith; that's why parents chose Catholic schools."
The government's recommendations, on the back of an independent review of PSHE, were widely welcomed by sexual health charities and religious groups involved in the review.
But secondary school heads regretted the government's decision to make PSHE compulsory, saying it was unnecessary and would make the curriculum "overcrowded".
Dr John Dunford, the general secretary of the Association of School and College Leaders, said: "Central prescription is increasing when it ought to be reduced.
"Regrettably, governments have a horrible habit of making more and more things compulsory and increasing the constraints on state schools.
He said compulsion was unnecessary as all secondary schools already have PSHE courses and it would make little difference.
"Experience suggests that once an item becomes compulsory, detailed prescription of what is to be taught, how, and for how long, is not far behind.
"This would be damaging to good and effective PSHE, which should always reflect the local context of the school."
Julie Bentley, chief executive of FPA, formerly the Family Planning Association, was pleased by the announcement: "This is a momentous decision. This move will dramatically improve the long-term health and well-being of our children and young people.
"Research shows that sex and relationships education helps young people delay the time they first have sex, and promotes responsible and healthy choices when they do become sexually active."
However, a minority of traditional family campaigners opposed the decision. Stephen Green, national director of Christian Voice, said the proposals would only "encourage experimentation" and contribute to the rise in teenage pregnancy and infertility.
He said: "We oppose this move completely. It can only encourage experimentation and will not teach about the rise in teenage pregnancy and infertility.
"Condoms only cover one part of the anatomy and do not protect against other infections carried on the skin that can cause infertility."
Christine Blower, acting general secretary of the National Union of Teachers, said: "Sex and relationships education is important for children and young people and should address all sensitive issues. This is all the more so given the levels of teenage pregnancy and the rise in STIs.
"However, there has just been a major review of the secondary curriculum and there is shortly to be a similar review in primary schools. Quite simply, space has to be made for PSHE in the school day, and sufficient training and necessary specialist staff made available."