Children as young as four could be forced to have sex education.
Government advisers want them to be taught about relationships and the names of body parts. They argue that starting sex education early will put youngsters off rushing into sex.
At the moment parents have the right to pull children out of classes.
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But charities FPA (formerly the Family Planning Association), Brook and the Sex Education Forum have told an official review, due to report next month, that the optout must end. Brook's Simon Blake said: "All the evidence shows that if you start sex education early - before puberty, before they feel sexual attraction - they start having sex later. They are much more likely to practise safe sex."
But Norman Wells, of Family and Youth Concern, said the plan would increase teen pregnancies and sexually transmitted diseases.
He said: "It's quite extraordinary to be calling on the Government to impose something on every child in every school that has no proven benefit whatsoever."
