The subjects are to become mandatory in all primary and secondary schools as part of a shake-up aimed at cutting Britain's high teenage pregnancy rate and steering youngsters away from drug and alcohol misuse.
The lessons will form part of a wider new personal, social and health education (PSHE) curriculum that is expected to be introduced by 2010.
The announcement follows reviews into sex and drugs education in school.
But one school leader said the move was "unnecessary".
John Dunford, general secretary of the Association of School and College Leaders (ASCL) 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."
Christine Blower, acting general secretary of the National Union of Teachers (NUT) said: "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 personal, social and health education in the school day, and sufficient training and necessary specialist staff made available.
"When Citizenship was introduced, it was bolted onto the secondary curriculum and many schools are still struggling to include it today."
Chris Keates, general secretary of the NASUWT teaching union said ministers' proposals to allow parents to opt out of sex lessons would made a mockery of the compulsory aspect of the classes.
Presenting the findings of the reviews, schools minister Jim Knight said Sir Alasdair MacDonald, head teacher of Morpeth School in Tower Hamlets, East London, will lead a review into the best way to make PSHE compulsory and consider where parents should be allowed to opt out.
Mr Knight said: "I think it's important for individual parents' views to be taken into account in some of these sensitive areas and their right to withdraw from parts of education in those areas that they do not feel comply with their moral views and beliefs and that they will be better dealing with in the home."
But Ms Keates said: "Placing a statutory requirement on schools will be rendered meaningless if every parent has the right to withdraw their child from sex and relationships classes.
"If it is important enough to be statutory then it is important enough for every child to receive it."
Under the new proposals, primary school children will learn about puberty and the facts of life from the age of seven.
From the age of five, pupils will be taught about topics such as the parts of the body, relationships and the effects of drugs on the body.
As pupils progress through school they will be given detailed information about contraception and sexually transmitted infections as well as the risks of drug and alcohol misuse.
Teachers will be given specific training into how to teach the subjects.
He said all schools in England will follow a "high-level programme of study", but faith schools have been promised supplementary guidance, allowing them to teach pupils about subjects such as contraception or homosexuality in a way that meets the beliefs of the school.
While today's announcement has been welcomed by some sectors, family campaigners reacted angrily and demanded the plans be scrapped.
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 the idea of teaching young children about sex is "a wickedness" from a Government that wants to see "a whole generation fornicating".
Norman Wells, director of the Family Education Trust, said the minister had gone back on a commitment to launch a full public consultation on any recommendations made by the review group before decisions are made.
He called it "very disturbing" that such a decision had been taken without reference to parents and groups representing parents' concerns.
He said: "There is no evidence to suggest that starting sex education in primary schools is going to reduce teenage pregnancy rates and improve sexual health."
But Gill Frances, chairman of the Teenage Pregnancy Independent Advisory Group, said: "Statutory PSHE is absolutely crucial in reducing teenage pregnancy, particularly for vulnerable young people, but all children and young people need equipping with the skills and knowledge to help manage their lives."
Shadow families minister Maria Miller said: "We support sensible sex and relationship education for children at an appropriate age.
"However, we oppose policies that would involve five-year-olds being taught about sex. We think that every parent must have the right to withdraw their child from such lessons."
Liberal Democrat children's spokeswoman Annette Brooke said the proposals were "long overdue".
She said: "Personal and social education is much broader than simply telling people about how and when to have sex. It must also cover emotional development and tackle issues around bullying.
"If properly delivered it can be used to empower our young people and enable them to make their own decisions without feeling pressurised."
Under current rules, schoolchildren must be taught the biological facts of reproduction, which usually happens in science classes.
Every school must have a sex education policy, but there is no statutory requirement for teaching about relationships and the social and emotional side of sexual behaviour.
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