Children should be taught about the “negative impact of online pornography” in the classroom, one of David Cameron’s main advisers has said.

Claire Perry, the Conservative MP, made the call as leading charities call for an overhaul of sex education.  

One leading UK children’s charity, the National Society for the Prevention of Cruelty to Children (NSPCC), warned that many young girls felt they have to “look and perform like porn stars” in order to be liked by boys.

The NSPCC called on the Government to update its “woefully inadequate” sex education guidance, which is 13 years old and contains no reference to the internet.

The charity said that the Prime Minister should urgently convene a group of experts, young people and teachers to rewrite the guidance, which is the only official teaching material available, to bring it into the 21st century.

Since the document was published in 2000, the rise of online pornography and social media means children are being exposed to sex more often and earlier than before, experts said.

Research by the NSPCC, commissioned by The Daily Telegraph, has shown that in the absence of meaningful sex education in schools, the internet is warping young people’s views about relationships.  

Almost a third of pupils believe pornography dictates how they should behave in a relationship, the survey of 601 secondary school pupils aged 11 to 18 showed.  

Seventy-two per cent said pornography should be talked about in sex education, suggesting that lessons are too focused on the mechanics of reproduction and lack meaningful discussion about the issues young people face online.

Other findings showed that schoolchildren are three times more likely to go online for advice or information on sex and relationships than they are to consult their parents or helplines.

Claire Lilley, a policy adviser at the NSPCC, said: “It’s natural for children to become curious about puberty and sex. If they are not learning what they need to at school or at home they will turn elsewhere, including to online porn.  

“What pornography teaches boys is that girls are for sexual gratification, while girls feel they have to look and perform like porn stars to be liked and valued by boys. This makes children vulnerable to being forced or pressured into behaving sexually.”

Ms Lilley said it was vital that teachers were equipped to help pupils interpret what they face online.

“The current sex and relationships education guidance which predates social media is woefully inadequate.

“Many children will have already been exposed to a vast amount of inappropriate, unrealistic and sometimes downright harmful pornography through the internet. This can warp their view of what is normal and acceptable sexual behaviour.

“To protect children from these damaging messages they must be taught about sex in the context of healthy, caring relationships and how to protect and respect themselves and others.”