Explicit age limits will appear for the first time in statutory guidance issued to schools in the UK covering relationships, sex education and health.

Children in primary school will not be taught sex education until they are in Year 5 — aged nine — while discussion about explicit sexual acts will not take place until children are 13.

Teachers will also be banned from giving children lessons about gender identity — the idea that children can have a sexual identity unrelated to their biological sex, adopt different pronouns, names and wear uniforms of the opposite sex.

The move represents an attempt to deal with concerns that some children are being exposed to sensitive information when they are not ready for it.

When children are first taught any sex education, in Year 5, when nine years old, it should be in line with the science curriculum, with a factual approach teaching children about conception and birth.

Parents will need to be consulted about anything that children are taught. The guidance is explicit that primary school children should not be taught about pornography.