"The National Curriculum science is the only compulsory part of sex and relationships in education in schools and must teach children about how their bodies work to prepare them for growing up and to protect them from harm, " it says.

BBC: Curriculum plan 'waters down sex education' experts warn

In a statement, Mr Knight said parents did not have the right to withdraw their children from the statutory sex and education part of the curriculum, only the optional PHSE element.

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Tory MP David Burrowes asked whether parents would be able to take their children out of RE, sex education and other classes in the curriculum where pupils were "taught about the value of same-sex marriage".

BBC: Maria Miller

Sex education also needs to be an integrated part of the national curriculum in every school not just an optional extra for interested schools.

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