Children as young as seven should get compulsory sex education lessons, the Liberal Democrats have said.
Lib Dem delegates voted overwhelmingly in favour of the move at the party's annual conference in Brighton.
Critics of the call said compulsion was wrong and parents should be trusted to decide for themselves whether their child should go to sex education lessons.
But supporters of the motion argued that the rise in teenage pregnancy and sexually transmitted diseases meant youngsters needed advice.
A bid by the party leadership to allow parents to withdraw their children from the lessons was defeated on the conference floor.
And delegates said the lessons should include information on contraception "at appropriate stages".
Ahead of the debate, health spokesman Evan Harris stressed that the education should be appropriate to the child's age.
"It would be ludicrous to suggest that condoms would be handed out to seven-year-olds," he said.
He said far from encouraging youngsters to engage in early sexual activity, properly directed education could actually delay children from becoming sexually active.
Dr Harris said the experience of countries like Sweden showed "clear information" at an early enough stage could reduce abortion rates and teenage pregnancies.
He added that far too high a proportion of girls in the UK reached puberty without being aware of what periods were.
'Loony Left' fears
Treasury spokesman Matthew Taylor argued that informing children in a manner appropriate to their age could help prevent them from being sexually abused.
"This is a policy approved by the NSPCC because to protect children they have to have an idea of what inappropriate behaviour is," he said.
"We are talking about teaching seven-year-olds to protect them."
During the debate, Lib Dem education spokesman Phil Willis said better sex education would
help reduce the spread of sexually transmitted diseases.
But he warned that making classes compulsory and including contraception advice risked being portrayed as a "loony Left" policy by critics.
"It's fundamentally illiberal to say to parents, often with a strong faith
or family belief, that they cannot withdraw their children from parts of the
programme they believe should be taught in a family setting," said Mr Willis.
The Department for Education and Skills currently recommends primary schools have a sex and relationship education programme, which should ensure children know about puberty and about how a baby is born.
But this is not compulsory. Schools are expected to consult parents on their policy on sex and relationship education, and parents have a right to withdraw their children from the lessons.
Lib Dem delegates have also backed a proposal to ban smacking children.
That motion called on the government to remove the "reasonable chastisement" defence and given them the same protection under the law on assault.
It also suggested introducing "help for parents with positive forms of discipline".