By Graeme Paton, Education Editor
Pupils will get basic classes in identifying body parts in the first few years of primary school.
In later years, they will be required to have more structured lessons about reproduction and relationships, a major review will recommend.
At secondary level, schools should improve the way issues such as civil partnerships and the importance of marriage are covered.
Teachers will also be given training in delivering lessons amid fears too many are embarrassed to discuss sex in the classroom.
The Government has already admitted that sex and relationship education across England is too "patchy".
However, the move will be opposed by family campaigners who accuse ministers of subjecting pupils to controversial issues before they are ready.
To allay concerns, ministers are expected to announce a consultation ahead of the implentation of the lessons on whether or not to give parents an opportunity to withdraw their children.
In a further move, ministers will also announce a radical shake-up of the way children are taught about the dangers of drugs and alcohol.
Jim Knight, the Schools Minister, who has led the review, insisted exposure to sex education before puberty reduced teenage pregnancy rates.
"It is important that we as a society allow better sex and relationship education in both primary and secondary schools without sexualising young people too early," he said. "It is right to share the responsibility between home and school."
At present, all primary and secondary pupils have to learn about the biology of reproduction in science.
In primary schools, pupils should learn about how animals and humans reproduce, but can limit lessons to the biology curriculum.
Schools can also cover the subject in personal, social and health education, although it is not a compulsory part of the National Curriculum.
In secondary schools, teachers must go further, covering issues such as relationships and sexually transmitted diseases. Lessons on civil partnerships and marriage are also offered at secondary level as part of PSHE but they are non-statutory.
Mr Knight said he had received "many strong representations" for making PSHE statutory at all ages to address the problem of poor lessons.
Speaking in the Commons, he said: "The international evidence suggests that teaching aspects of sex and relationship education before puberty has a positive effect on such things as teenage pregnancy rates. Clearly, that has to be done with a high degree of sensitivity and... the involvement of parents, with children reaching puberty at different ages. We must ensure not only that, as a society, we are comfortable with the level of detail and of education that people receive during sex education, but that we are strong on relationship education."
Leading charities including the Sex Education Forum and Brook, the sexual health advice service, which have taken part in the Government review, have already called for lessons to be compulsory in all schools.
It follows the publication of Government figures earlier this year showing that the number of abortions in girls under 16 last year rose 10 per cent to 4,376.
Norman Wells, director of the Family Education Trust, said making sex education mandatory would "seriously undermine parents".
New-style lessons on drugs and alcohol lessons will also be overhauled.
Primary school pupils will be given warnings on avoiding medicines and prescription drugs left in the home - as well recognising the difference between soft drinks and alcohol.
Stephen Burgess, national director of Life Education Centres, the health charity, said: "If we want to make a real and lasting difference to teenage drug and alcohol misuse, we must reach them early – at primary school. Then, as they reach adolescence and are most at risk from peer influence, they can make informed decisions based on fact rather than hearsay."
*Health officials were forced to apologise after sending letters to children as young as nine, demanding they are tested for sexually transmitted diseases.
Primary school children were also offered the chance to win an iPod if they attended a clinic for testing. A primary care trust in Harrow blamed an administrative error.
Geraldine Smith, the Labour MP for Morecambe and Lunesdale, told the Telegraph: "To start sex education at primary school is to rob young children of their innocence. I know children seem to grow up faster these days but to start formally teaching them about sex education would be quite wrong and would encourage under-age sex. Being exposed to this sort of thing at such an age would put an awful lot of pressure on very young children."