Sex education initiatives could fail because not all teachers have the time, training or desire to give the compulsory lessons, according to a new report.
Researchers at Birmingham University found that teenagers in British secondary schools do not know enough about sexual health and contraception as a result.
During a year-long study, more than 1,500 pupils aged 11 to 16 from rural and urban locations were questioned about their attitudes, behaviour and knowledge.
But the study revealed teachers and school nurses are still ill equipped, and sometimes reluctant, to teach the compulsory subject.
It also revealed that classroom teachers find it difficult to effectively inform pupils on subjects like sexually transmitted infections and emergency contraception, although their general sexual health knowledge is good.
Jo Westwood, PhD student from the university's School of Health Sciences, who led the study, said: "This research suggests teachers are being expected to contribute to secondary school sexual health education programmes at a time when they not only have insufficient knowledge to provide young people with adequate sexual health education, but they are being expected to participate in a subject they really do not feel prepared to teach and in many cases, would prefer not to teach."
Ms Westwood believes Sex and Relationships Education (SRE) programmes will only work if teachers are given upto-date training and want to teach the subject.