The study said several of the teens admitted putting the condom on too late or taking it off too early.

The three most common reasons for condom use were to prevent pregnancy, avoid making a mess and prolonging sex -- avoiding sexually transmitted diseases was barely mentioned.

Of the 1,400 teens in the study, 373 said they had used a condom in their most recent sexual experience. Six percent said they put the condom on after vaginal penetration and an equal number said they continued penetrative sex after removing it.

"There was an element of ignorance of how they are supposed to be used," Nicole Stone of the Centre for Sexual Health Research at Southampton University in southern England told Reuters. "We need to correct that."

The study, published in the journal Sexually Transmitted Infections, was prompted by the anomaly that while teenage pregnancies are falling, rates of sexually transmitted disease are rising.