Sex education is failing in many schools, it was claimed today, with nearly half of British young people not being taught about teenage pregnancy.

A Youth Parliament survey also found that a similar proportion of teenagers said they would not know where to find their local sexual health clinic.

In the survey of more than 20,000 young people in England, more than half rated the teaching in school as poor, very poor or average. Only a quarter said it was good.

The findings prompted leading children's organisations, sexual health experts and MPs to call on the schools minister, Jim Knight, to make relationship teaching a statutory part of the national curriculum.

They said in a letter to the Times that the paucity of good teaching was contributing to the country's sexual health crisis.

Young people are bringing their findings and proposals on sex and relationships education to the minister this week. "We urge the government to listen to what they have to say," the letter said.

"These [survey] figures ... may go some way to explaining disproportionately high rates of teenage pregnancy and sexually transmitted infections in this country.

"We believe that all children and young people are entitled to receive sex and relationships education (SRE) as part of the statutory provision of personal, social, health and economic education (PSHE education) in schools. No school should be able to opt out ... including primary schools, faith schools and academies."

The letter's signatories included Nick Partridge, the chief executive of the Terrence Higgins Trust, Steve Sinnott, the general secretary of the National Union of Teachers, and Dame Mary Marsh, the chief executive of the NSPCC.

At present, sex education is a statutory part only of the science curriculum for 11- to 14-year-olds.

Figures from the Health Protection Agency revealed that among 16 to 19-year-olds, diagnoses of herpes rose by 13% and those of genital warts increased by 6% between 2005 and last year.

Government figures also showed that the UK still had the highest levels of teenage pregnancy in western Europe. For every 1,000 births between 2000 and 2005, 27 were to under-19s.

In a statement last night, Knight said: "We are clear that parents do not have the right to withdraw their children from the statutory sex and education part of the curriculum, only the optional PHSE element.

"Our guidance is also clear that schools should teach about the risks of unprotected sex and direct young people to local services where they can access contraceptive and sexual health advice. In that respect, the UK Youth Parliament's findings are disappointing.

"Nevertheless, our teenage pregnancy strategy is working - teenage pregnancy rates are at their lowest for 20 years. We made a commitment to reduce the teenage conception rates and to improve the quality of life for all young people. We are delivering on that."