Schools in England will be banned from teaching sex education to children under nine, in new government guidance expected to be published on Thursday.
The BBC has not seen the new guidelines but a government source said they also included plans to ban any children being taught about gender identity.
Head teachers have said there is no evidence of a widespread problem with age-inappropriate materials.
One union has said the review is "politically motivated".
The statutory guidance on relationships, sex and health education (RSHE) - which schools must follow by law - is currently under review.
The government believes clearer guidance will provide support for teachers and reassurance for parents, and will set out which topics should be taught to pupils at what age.
The BBC has not seen the new guidelines but a government source said they also included plans to ban any children being taught about gender identity.
Head teachers have said there is no evidence of a widespread problem with age-inappropriate materials.
One union has said the review is "politically motivated".
The statutory guidance on relationships, sex and health education (RSHE) - which schools must follow by law - is currently under review.
The government believes clearer guidance will provide support for teachers and reassurance for parents, and will set out which topics should be taught to pupils at what age.
Plan to ban sex education for children under nine
New guidance for schools in England is expected to be set out on Thursday.
www.bbc.com