Children as young as seven should be given compulsory sex education at school, Liberal Democrats said.
They argue the move is necessary to reduce teenage pregnancy and combat sexually-transmitted diseases.
Tory leader Iain Duncan Smith branded the policy "offensive" but a Liberal Democrat spokesman denied the policy would mean condoms for seven-year-olds, insisting teaching of contraception should come only "when children are better prepared".
"Children as young as 12 are now sexually active. We must teach them the dangers of inappropriate relationships," he said.