Children as young as four will be taught about sex and relationships as well as the dangers of the internet.

The new lessons in primary schools are set to be announced alongside moves to teach secondary school pupils the risks of online pornography and sexting.

The classes will be compulsory for schools under an amendment to the Children and Social Work Bill.

Parents would, however, be able to opt their children out of the Sex and Relationships education, under plans to be announced by Justine Greening, the education secretary.

David Burrowes and Maria Miller, the Conservative MPs, tabled the amendment, saying that the consequences of failing to teach children about sex and relationships caused “physical and emotional harm”.

They warned that teenagers were “developing a sense that sexual harassment and sexual violence are acceptable behaviours and learning social norms that are carried through to adult life”.

At present only council-controlled secondary schools are required to teach about sex in biology lessons — academies and free schools are exempt.

An investigation in The Times last year found that thousands of schoolchildren had been caught sharing sexual imagery.