Forty-nine per cent of sexually active 13 to 16-year-olds claim that they do not always use a condom when having sex, while 45 per cent said that they did not regularly use any contraception at all.
And almost 25 per cent of girls and 20 per cent of boys say they have no intention of using a condom within the next six months.
Louise Wallace, of Coventry university, questioned 3,800 teenagers about their sexual experience and activity. She found that those pupils who were not sexually active were far more likely to have positive attitudes towards contraception. Eighty-five per cent said that they intended to use condoms every time they had sex.
"Children have a lot of difficulty getting hold of condoms," she said.
"Often, they can't afford them. We need to tell them where they can find contraceptives and make them available for free in schools.
"It ought to be all right for boys and girls to carry condoms around. It should be welcomed if they have condoms in their schoolbag. In some schools, that might not be seen as a good thing."
Professor Wallace insists that this demonstrates that simply teaching the facts of condom use will not guarantee actual usage. Pupils, she says, know that they should be using them. The problems occur when their partner does not agree.
She said: "Their use is part of the process of sex. Girls should feel able to supply them if partners do not."
Equally, she said, teenagers need to be made aware of emergency contraception. "They don't know where to get it or when it's safe to use.
"And there are other fears. What will people think of me if I use it? Is it a form of abortion? Will I be judged for taking risks?
"Yes, children are taking risks. But there's such a high rate of unprotected sex. The point is that we need to find ways to stop it happening."