The government is being urged to change the law so that sex education books in schools are vetted by film censors.

Conservative MP Andrea Leadsom says some of the material being taught to children as young as five is "extraordinarily inappropriate".

She wants books and videos used for sex education to be given a rating by the British Board of Film Classification before they are used in schools.

A review of the national curriculum is looking at the future of sex education.

During a Westminster Hall debate, Ms Leadsom - MP for Northamptonshire South - said many adults were "horrified" when they found out what children were being taught about sex.

She also wants the law changed so that parents actively have to "opt in" to lessons, rather than "opt out", as is currently the case if they have objections.

According to a report published by Ofsted last year, a quarter of schools in England are not providing good enough lessons about sex, relationships and health.

Ms Leadsom, the MP for South Northamptonshire, said: "I've seen cartoons of two people engaged in sexual activities with the caption 'Here are some ways mummies and daddies fit together', others depicting two cartoon characters locked in an intimate embrace, accompanied by a vivid explanation, using sexual terminology of the act of intercourse.

"As well as cartoons I've been shown a video of two people engaged in intercourse, with a child's voice over the top, saying, 'it looks like they're having fun'."

The MP recommended that books and videos used for sex education in England be given a rating by the British Board of Film Classification before they are bought by schools and local authorities.

She also said the law should change so that parents actively have to "opt in" to lessons, rather than "opt out", as is currently the case if they have objections.

There had to be more emphasis on building loving and lasting relationships.

Schools minister, Nick Gibb, said all sex education material used in state schools was scrutinised to ensure it set the "right tone".

The education secretary had set out statutory guidelines for schools and councils to follow, he added, which would ensure that "inappropriate" content would not be used.

For Labour, shadow schools minister Kevin Brennan said it was important for Ms Leadsom to name the schools using the material she had referred to.

But she said this was not necessary for the purposes of the debate.