The teaching of sex and relationships in primary schools is not progressing fast enough to prepare children for the earlier onset of puberty, the Independent Advisory Group on Teenage Pregnancy will warn ministers in its annual report.
The advisers have been encouraged by a 10% reduction in the teenage conception rate since 1998, and they attribute part of this improvement to more confident teaching of personal, social and health education (PSHE) in secondary schools.
But the government-appointed group is expected to say: "We are disappointed this new confidence is not reflected in primary schools."
It will call for PSHE to be made part of the statutory curriculum at all key stages of education from five to 16, with regular checks by Ofsted, the schools inspectorate, on the competence of teaching.
Under the current rules, most state schools provide PSHE, but only the study of citizenship in secondary schools is compulsory. Ofsted reports on the emotional, spiritual and moral development of pupils, but there has been no thorough study of sex and relationship education.
Gill Frances, deputy chairwoman of the advisory group, said: "We know this is not properly assessed across all schools." But it was important to start education about sex and relationships in the early years at primary school.
Encouraging children to start talking about feelings and relationships developed emotional skills that helped them to avoid teenage pregnancy, sexually transmitted infection and drug taking, she said.
The advisory group will ask ministers to give statutory force to sex education guidelines prepared by Ofsted. They say pupils by the age of seven should be able to compare the external parts of the human body, share their feelings and use simple rules for resisting pressure from strangers.
By 11 they should be able to express opinions about relationships and bullying, recognise their changing emotions, discuss moral questions and know how to resist unwanted physical contact.
They should understand the physical changes that take place in puberty, the need for love in stable relationships and the safe routines needed to avoid the spread of viruses including HIV.
Ms Frances said the report would advise ministers to leave schools some discretion over the pace of the sex curriculum: "There is no point in pushing schools to do more than parents and the local community can stand."
The advisory group is expected to call on the government to do more to change the behaviour of hard-to-reach groups.
Proposals include a national information campaign targeted at boys and young men, particularly from black and ethnic minorities, and an advert to ensure under-16s know they have the same rights to confidentiality as adults when they seek advice.