Lessons on homosexuality will be mandatory in schools throughout England but parents can withdraw their child if they wish, the Government has said.
The requirement will also apply to faith schools but they will be allowed to cover the material in a way which accords with the school’s religious ethos.
The lessons are set to become part of the compulsory curriculum in 2011.
Schools will also be obliged to teach about the importance and value of marriage.
The plans are part of the Government’s wide ranging overhaul of sex education. A four month consultation on the plans is expected to start soon.
The Government’s announcement follows a review covering sex education headed by Sir Alasdair Macdonald.
In recommendations accepted by the Government, Sir Alasdair’s report said personal, social, health and economic (PSHE) education should become part of the compulsory curriculum.
The current programme of study for PSHE says children should have lessons on same sex relationships from the age of eleven.
A review of the primary curriculum by Sir Jim Rose, due to be published later this week, will also consider how PSHE should best be delivered to younger children.
At the moment schools are not obliged to teach PSHE but many choose to do so.
Parents currently have the right to withdraw children from sex and relationship lessons and Sir Alasdair recommends keeping this opt out in place.
The Government says it will accept that recommendation for the time being, but it will keep the parental opt out under constant review.
The decision to make PSHE compulsory in schools has been criticised by family and faith groups.
Norman Wells, of the Family Education Trust, said: “Making PSHE a statutory part of the national curriculum could be used as a vehicle to promote positive images of homosexual relationships.
“It is difficult to see how teaching children as young as 11 about same-sex relationships and civil partnerships fits in with a study of personal wellbeing, and many parents will be very concerned about the prospect of such lessons being imposed over their heads.”
Simon Calvert, of the Christian Institute, said that “pressing the virtues of homosexuality” could lead to more experimentation, which could be “harmful” to children.
He said: “What we don’t want to see is vulnerable young people being exploited by outside groups which want to normalise homosexuality.”
Teaching unions have expressed concern about the added workload.
The Association of School and College Leaders said it disagreed that PSHE should become compulsory, but called Sir Alasdair’s recommendations “sensible”.
General secretary, Dr John Dunford, said: “it is difficult to see why the government wants to turn this into a statutory requirement.”
Christine Blower, acting general secretary of the National Union of Teachers, said some schools were still struggling to deliver Citizenship, which had been “bolted on” to the curriculum.
Both representatives of Church of England and Catholic schools welcomed the right of schools to maintain their own values in teaching the subject.
The Catholic Education Service said such lessons should be taught “in line with the wishes of parents and the ethos of the school” – including teaching in “age-appropriate ways”.
But Terry Sanderson of the National Secular Society said it was “unfortunate” that the Government is allowing faith schools to teach sex education in line with their religious ethos.