Compulsory sex lessons for primary school children as young as five are to be backed by the government's official advisers on sexual behaviour in an unpublished report obtained by The Observer. If accepted, the proposals would be the biggest shake-up in sex education in schools in England and Wales.

The document says the current system for sex lessons, which are mostly optional, is unfair, confused, damaging to pupils' health and development and partly responsible for Britain having the highest rate of teenage pregnancy in western Europe. At present all pupils get basic biological information, but those at some schools are also given details about subjects such as contraception and sexually transmitted infections.

A joint report from the Government's independent advisors on sexual health and teenage pregnancy recommends that detailed knowledge about sex should become a routine part of all pupils' education and points out that adopting such an approach makes young people better able to handle sexual issues. The 42 advisors include senior doctors, experts in sexual behaviour, specialists in bringing up children, nurses, and leading academics in the field.

They want ministers to make Personal, Social and Health Education (PSHE) a statutory subject in all primary and secondary schools in England and Wales. Certain schools provide PSHE to help prepare their students to understand the adult world of sex, alcohol, drugs and bullying.

The Sex and Relationship Education (SRE) element of PSHE includes much more in-depth discussion about sexual activity than the factual reproductive biology all pupils cover in science lessons as well as tuition on how to deal with pressure from friends or partners to have sex, where to get contraception and how infections such as chlamydia and genital warts are passed on.

The report, 'Personal, Social and Health Education in schools: Time for Action', has been compiled by the Independent Advisory Groups on Sexual Health and Teenage Pregnancy, which advise the Department of Health and the Department for Education and Skills.

Labour peer Joyce Gould, who chaired the inquiry, said last night that the government should make PSHE a statutory part of the national curriculum in order to tackle the high number of teenage girls becoming pregnant, the rising levels of sexually transmitted infections and widespread ignorance among young people about sex.

Gould denied the group's proposals would encourage promiscuity. 'Some people will say that if you don't tell them about it, they won't do it. But real life shows that's not the case. More and more young people are having sex at a younger age.'

If implemented, primary school children would be taught mainly about emotional issues such as relationships and friendships, with older ones starting to learn about puberty. Only secondary students would discuss sexual activity and its potential pitfalls.

Gill Frances, the acting chairwoman of the teenage pregnancy advisors, said SRE was vital to help pupils understand complicated sexual issues. 'Young people are growing up in an increasingly sexualised society, where there are mixed messages about sex. The result is that they end up confused because they don't understand what sex is all about.'

Frances said that mandatory PSHE would make many young people more likely to postpone their first sexual experience, and more confident at engaging with the opposite sex. The report shows that teenage pregnancy in Liverpool, Bradford and Hackney, in east London, fell after local schools introduced PSHE and SRE.

Members of the two advisory groups believe that junior ministers at the DFES and DoH, such as Beverley Hughes, the minister for children, young people and families, and public health minister Caroline Flint, are sympathetic to their plea. But education secretary Ruth Kelly, a devout Catholic, is thought likely to oppose such a dramatic extension of pupils' knowledge about sex. The growing number of faith schools could also make implementing PSHE difficult. Parents can currently remove a child from SRE if they are unhappy with the content.