The lessons will begin in more than 200 schools in the New Year, using teaching materials produced by the National Society for the Prevention of Cruelty to Children.
Never before have children so young received such explicit teaching and school authorities are bracing themselves for the inevitable parental complaints.
According to Catherine Howard, an NSPCC education adviser, the new classes will include discussions about which parts of the body children considered to be "private". Teachers will explain to children the differences between being examined by a doctor and sitting on a parent's knee or being threatened by an adult.
"Teachers will talk to the children about what they would do if somebody touched them in those places or in a way that made them feel uncomfortable, or if they were asked to do something that made them feel funny," she said.
A spokeswoman for the Department for Education and Employment said the teaching materials would be used only with the consent of teachers, parents and governing bodies. It was up to schools to decide at what age to raise the question of sex abuse. "It is very unlikely to happen at the age of six," she said.
The proposals were criticised by the traditionalist Campaign for Real Education last night. "While child abuse is a terrible thing, I do not think it is something that should be brought up as a matter of course in schools," said the campaign's spokesman Nick Seaton.
"Children know these things instinctively without being taught and it is up to teachers to have a quiet word with a child they are worried about. The danger is that bringing all these things into the open desensitises children and destroys childhood."
Members of the Islamic community and the Conservative education spokesman, John Bercow, expressed reservations about how the subject would be taught. "Talking to children about the private areas of their bodies is going to focus attention on them and could put ideas into their heads," said Mohamad El Sharkawy, imam of the Regent's Park mosque in London. "They should be taught how to protect themselves from people who are trying to abuse children but not in such a direct way."
The NSPCC has consulted with local authorities and teacher organisations. The National Union of Teachers has endorsed the plans. Supporters of the approach cite the extent of sexual abuse in the UK. Up to two children die from abuse or neglect every week and more than a quarter of all rape victims are under 16, including 4 per cent who are under nine.
"Nothing is going to change the widespread nature of child cruelty in this country unless we identify and support vulnerable children and target the young parents of the future through the school environment," said Mrs Howard. "Children who are being abused very often do not realise what is happening to them. In addition, teachers have told us they feel they can't spot the signs of abuse and even when they do, they are worried about what action to take."
According to Marjorie Orr of Accuracy About Abuse, the NSPCC approach will help to prevent abuse that could otherwise continue for years because a child is unable to speak out.