Almost a fifth of the sexually-active students in the study used no protection the first time they had sex, while nearly half reported having had unprotected intercourse at some time in the past. Of those, one third of young women had subsequently taken emergency contraception in the form of the morning-after pill.

The survey of 16 to 20-year-olds, carried out by medical researchers at Keele University, revealed high levels of ignorance over contraception, particularly among young men.

The study finds no evidence that levels of sexual activity among teenagers are as high as some fear. Of the 1,281 students interviewed, slightly more than half said they had never had sex, while of the 49 per cent who had, the commonest starting age was 16.

Gina Milsom, who carried out the research at two Stoke-on-Trent sixth-form colleges with colleague Dr Ruth Chambers, said she had been “surprised and relieved” to discover so many were thinking twice before experimenting. “There is a common perception, not least among teenagers themselves, that they are all ‘at it’,” she said. “Clearly that is not the case.”

But she highlighted evidence in the study showing the majority of teenagers could not remember any lessons from schooldays on relationships and life choices, though they recalled learning the biological facts of reproduction.

That ignorance, combined with the fact that the first year after leaving mainstream education was often a time of experimentation, was an argument for colleges to consider providing students with far more guidance, she said.

“Colleges and schools with sixth forms should be encouraged to include sex education. While educationists will argue that students are in college to be educated for their working life, it is just as important to try to improve their health and well-being.”

Judith Norrington, curriculum officer at the Association for Colleges, said colleges did what they could within cash constraints.