Announcing the guidance, Mr Sunak said: “Parents rightly trust that when they send their children to school, they are kept safe and will not be exposed to disturbing content that is inappropriate for their age.

“That’s why I was horrified to hear reports of this happening in our classrooms last year.

“I will always act swiftly to protect our children and this new guidance will do exactly that, while supporting teachers to teach these important topics sensitively and giving parents access to curriculum content if they wish.”

Ms Keegan said the new guidance had been motivated by a request from teachers “to provide more clarity” on age-appropriate sex education, but she had also seen reports of “campaign groups’ or lobby groups’ materials” being used in classrooms.

She said she had received evidence of lesson slides including “things like choosing lots of different genders and identities and saying which ones of these are gender identities – the spectrum. The sort of, ‘it can be a spectrum, it’s fluid, you can have different genders on different days’ or ‘there’s 72 of them’. That kind of thing”.

Ms Keegan added: “Parents are the first educators of their children, so there should be transparency and they should be able to see the materials, and we’ve made that crystal clear.”

The education secretary admitted to BBC Radio 4’s Today programme that she did not know how widespread the issue was as “it’s not something that we’ve gone and done a particular survey of”, but said she did not think it was common.

Other elements of the guidance include: