Expanding family planning services has had little impact on teenage pregnancy rates, research suggests.
The government says sex education has improved
A study by a Nottingham economist says providing extra clinics may even lead to increase in rates of sexually transmitted infections (STIs).
Professor David Paton, was examining how people behave with a "safety net".
He suggested knowing there was easier access to the morning after pill and treatment for STIs may encourage teenagers to take more risks.
The government introduced a Teenage Pregnancy Strategy in 1999, aimed at reducing conceptions in teenagers under-18.
Figures for 2002 showed a slight increase in the rate after several years of decline.
Since 1999, figures from genitourinary clinics show that diagnosis rates of STIs such as chlamydia and gonorrhoea have increased by 24%.
Professor Paton, who is based at Nottingham University Business School, looked at data from health authorities across England for 1998 to 2002.
Professor Paton told BBC News Online: "The improvement in services is not cutting teenage pregnancy rates.
"Overall, it is not causing them to go up - but it is also not causing them to go down.
"Increasing family planning provision is having a significant impact on STI diagnosis rates - but unfortunately in the wrong direction."
He said the existence of more clinics may mean that infections which previously would have gone undetected were picked up, but said that factor would not account completely for the increase in STI diagnosis.
Professor Paton added: "What happens when you provide these services is that you hope young people who are having sex will use the services and be less likely to get pregnant.
"But if family planning is more widely available, teenagers may be having sex when they wouldn't otherwise have done so.
"For example, if the morning after pill is available in school on a Monday morning, they may be more likely to have sex at the weekend."
Professor Paton, who is a member of the Society for the Protection of the Unborn Child, said the aim of family planning services should be to delay the age at which teenagers begin having sex, so that fewer then fall pregnant or develop an STI.
A Department of Health spokesman said: "The number of family planning sessions aimed at young people has increased as a direct response to wanting to tackle high levels of teenage pregnancies and increasing STIs.
"In turn, increased provision will have created greater opportunities for raising awareness of STIs, resulting in increased screening and testing.
"This will have contributed to higher rates of STI diagnoses not higher rates of infection."
He added: "Increasing rates of STIs and high levels of teenage pregnancy have complex underlying causes, requiring multifaceted prevention solutions.
"Increasing and improving access to contraception and sexual health advice services is one essential part of the Government's Teenage Pregnancy and Sexual Health Strategies which are aiming to improve young people's sexual health.
"In addition we are also helping young people to resist pressure to have sex, improving sex and relationship education to increase their knowledge and skills, and supporting parents to talk to their children.
"Since 1998, the baseline year for the Strategy, there has been a 10% reduction in the under 18 conception rate."