Schoolchildren are being told that having sex at 13 is a normal part of growing up.
Government-backed guidelines say that having sexual relationships when children reach their teens represents a 'safe and healthy' part of growing up.
They have been produced by the national charity Brook as part of a plan endorsed by the Department for Education to improve standards of advice given to pupils.
The 'traffic light tool' lists green, amber and red behaviours that teachers and other professionals should spot among school-age children.
For 13-to-17-year-olds, normal behaviour includes taking an interest in pornography, having sexually explicit conversations, using the internet to chat online and consenting to oral or penetrative sex with the same or opposite gender.
It also says that 'choosing not to be sexually active' is a 'green behaviour'.
However, Sarah Carter, trustee of the Family Education Trust, told a committee of MPs that the guidance was illegal, adding: "The law states that young person should wait until they are 16 at least.
"That's awfully unlawful behaviour, and so quite often what's taught in [sex education] isn't always lawful."
The DfE insisted the guide was 'not intended for use in planning sex and relationships lessons'.
A spokesman said: "No teacher should ever encourage or condone behaviour which is unlawful or risky.
"Good quality, relationship education is an important part of preparing young people for life in modern Britain and our statutory guidance makes clear that it must be taught in an age appropriate way.
"Schools should ensure young people develop positive values and a moral framework that that will guide their decisions, judgments and behaviour."
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