Children as young as four should be given advice on the dangers of pornography, according to headteachers.

Delegates at the National Association of Head Teachers’ (NAHT) conference in Birmingham warned that children were being robbed of their innocence by sexually explicit material on the internet as well as on television, often being shown before the 9pm watershed.

The call came as a survey of parents showed four in 10 would like their five-year-olds given lessons on dealing with pornography, at the same time as they learn how to use the internet.

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Amanda Hulme, headteacher of Claypool primary school in Bolton, said “completely inappropriate themes” such as rape, murder, domestic abuse, child abuse, self-harming, bulimia, alcoholism, prostitution and drug abuse were now being aired well before the watershed.

She singled out Britain’s Got Talent, saying it had allowed Sandra Broadbent to sing “You were lying there butt naked with a woman between your thighs” at 8.10pm followed by a chorus of “kiss my arse”.

The ITV soap Emmerdale also came under fire for allowing one character, Cain Dingle, to shout “I’ll kill him and the slag I catch him with” at 7.20pm.

On internet pornography, Steve Watkins, headteacher of Millfield primary in Leeds, said schools needed to talk to nursery-aged children about “things that shouldn’t be there” if they accessed material by mistake.

A survey of 1,000 parents by the NAHT showed more than four out of five were happy for their children to receive lessons about the dangers of pornography. In addition, 88 per cent felt sex education lessons should be compulsory in schools.

The parents were split over when the lessons should start, though, with 42 per cent saying five- and six-year-olds needed guidance. However, more than half (51 per cent) felt the lessons should not start until children were in their teens.

They were united, though, in supporting the idea that all equipment offering internet services should have a default block on pornographic websites requiring users to “opt in” rather than “opt out”; 90 per cent of respondents agreed.

Russell Hobby, general secretary of the NAHT, added: “Young people must be protected from pornography and children should receive appropriate guidance as part of sex and relationships education.”

On TV violence, Ms Hulme said heads should “at least fight to keep the watershed in place”.

“The perception is childhood has already been reduced from 16 to 12 and a third of the people surveyed [by Netmums] felt that childhood was over at 10,” she added. “How far are we prepared to let this go? – eight? six? I already have to speak to children in year two (six- and seven-year-olds) about inappropriate sexual language. This has to stop.”

Meanwhile, parents are signing children as young as eight illegally on to Facebook, said Mr Watkins.

He revealed the majority of his 10- or 11-year-olds (24 out of a class of 33) had been signed on – despite the fact that Facebook rules say they should not do so until they are 13.