Controversy over three sisters who had babies in their early teens has prompted renewed calls for more comprehensive sex education to be taught in schools as a compulsory part of the curriculum.

Yesterday, it emerged that the three girls from Derby became pregnant when they were aged 12, 14 and 16. Their mother, Julie Atkins, publicly blamed their school for not teaching them sufficient sex education.

Today, the Family Planning Association (FPA) said the case reinforced its campaign to make compulsory in schools sex and relationship (SRE) lessons, which deal with the emotional as well as the physical side of sex.

"We have been campaigning a long time for compulsory sex education. It's really, really important that young people are informed so they can protect themselves and make informed choices," said a spokeswoman for the FPA.

She said that while some schools taught pupils well-rounded lessons that covered all areas of relationships as part of their personal, social and health education (PSHE) programmes, the provision across the country was patchy.

"That's not the norm. Most schools don't give it the time or resources it should have," she added.

Jan Barlow, the chief executive of Brook, the sexual health charity for young people, said tackling the inadequacy of SRE in many schools by making it a compulsory part of the national curriculum must be a priority if Britain was to reduce its teenage pregnancy rate.

Britain has the highest number of teen pregnancies in western Europe and the cases of sexually transmitted diseases (STDs) among this age group have almost doubled in the last 10 years.

"Comprehensive sex and relationships education, combined with access to confidential sexual health services for young people, has been shown to encourage young people to delay having sex, and to use contraception when they do become sexually active," she continued.

However, both organisations maintained that sex education was not the sole responsibility of schools, or the only factor that needed to be addressed to tackle the rising number of pregnancies and STDs.

"Other factors that influence young people's sexual behaviour need to be addressed as well," added Ms Barlow. "Parents are an important source of information on sex and relationships for many young people and many of them would benefit from guidance on how to talk to their children about these issues.

"It is also vital that the educational and employment opportunities of young people from disadvantaged backgrounds are improved. As the government's own report on teenage pregnancy said of young people who can only see a future on benefits, 'put simply, they see no reason not to get pregnant'."

The FPA offers advice on its website on how parents can tackle the subject with their children. "Lots or parents want to get involved, but do not know how to go about it," said the spokeswoman. "Things change over the years and it's a difficult subject for some people to talk about."

Each school is required to prepare a policy on SRE. Under the national curriculum, schools are required to teach the biological facts of sex, and those about STDs, as part of their compulsory science lessons, which start in key stage 1.

Lessons that touch on the emotional side of the subject - relationships and morals - are taught under PHSE. Parents currently have the right to withdraw their children from any sex education lessons not included in the national curriculum.

A spokesman for the Department for Education and Skills said: "Good quality sex and relationship education is important and that's why the teaching of SRE is compulsory in secondary schools, and discretionary in primary schools. "All schools should have a policy for sex and relationship education drawn up by headteachers and governors in consultation with parents and having regard to the department's SRE guidance issued in July 2000."

He added: "We recommend that SRE is rooted in the framework for personal, social and health education in order that young people can learn to respect themselves and others and develop loving, caring relationships." The department's website also points out that the teaching of sex and relationships is a "partnership" between schools and parents and offers advice on tackling the subject at home.

In January an Ofsted report criticised the poor quality of PHSE provision in some schools. Inspectors found schools taught the facts of sex during these classes rather than teaching an understanding of how to deal and negotiate relationships.