Children as young as five should be taught about lesbian, gay and transgender issues in every subject, according to guidance issued to primary schools.

The voluntary guidance, from the campaign group Stonewall, has been launched to coincide with new relationships and sex education (RSE) lessons that will come into effect next September. Although parents will be allowed to remove their children from lessons focusing on sex, the relationships content is compulsory.

In design and technology lessons, teachers should use the rainbow flag to help 5 to 7-year-olds understand the meaning of colours, according to Stonewall, which says teaching about LGBT families and people must be “embedded” across school timetables. Another suggestion is that same-sex couples be used to teach children to count in maths lessons,