The study comes as the British Pregnancy Advisory Service (BPAS) prepares to give out the morning-after pill to women before they need to use it.

Currently, the pill, which is 75% effective in preventing pregnancy, is only available in the UK from doctors or family planning clinics after a woman has had unprotected sex.

It has to be taken within 72 hours of having unprotected sex, but BPAS say research shows it is 50% more effective if taken within 12 hours. That is why they want to make it more easily available.

Opponents argue that the move will encourage unprotected casual sex and that regular doses of the pill - which contains the equivalent of one week's worth of combined low dose pill hormones - could be harmful to health.

Valerie Riches, director of Family and Youth Concern, said: "It is saying that you can have casual sex and it might cost you �10 but you can bail yourself out."

The Finnish study of 52,700 teenagers aged 14 to 18 found that most girls had only used the pill once.

Writing in the British Medical Journal, the researchers say the argument that intensive sex education and easy access to contraceptives increases sexual activities does not appear to be true.

Social stigma

The BPAS said the results showed sex education did not increase sexual activity, but helped prevent pregnancy.

It said they were backed by a pilot study in the Lothian region of Scotland which showed that women given easy access to the morning-after pill tended to use it appropriately.

Half of the women who took part in the trial were given emergency contraception to take home and half were not.

Although women given the pill beforehand used it more than those not given it, they only used it "when they needed it instead of just crossing their fingers", said the spokeswoman.

She said many women were put off using the pill because it was difficult to get it quickly and there was a social stigma attached to it.

"It is associated with rampant, unprotected sex and some people still think women should pay for that," she said.

She called for more education about the pill, saying studies showed many British women did not know how to use it or where to get it.

The BPAS initiative, which begins on Friday, will allow women anywhere in the country to ring a national action line on 0845 7304030, book an appointment to see a doctor and get the pill.

It is part of a campaign to make the morning-after pill available over the counter.