Fertility experts from the World Health Organisation say women who delay taking contraception such as the "morning after" pill by 12 hours are 50% more likely to become pregnant.

The risk increases the longer women delay taking action.

The researchers used data on around 2,000 women who had taken part in a comparison of two different types of emergency contraception. Of these 38 became pregnant.

Delaying the dose

There are several types of "morning after" pills which can be taken for up to 72 hours after intercourse.

Some "morning after" pills work for 72 hours

Writing in The Lancet, the researchers, led by Dr Gilda Piaggio, said: "With both ... regimens, the earlier the treatment begins, the more effective it is. In the world's largest randomised controlled trial of these methods, delaying the first dose by 12 hours increased the odds of pregnancy by almost 50%."

The study confirms previous WHO research, published last year.

The Family Planning Association said it would still advise women to take action for up to 72 hours after intercourse.

"Women should not think they have to carry on with the pregnancy. They should seek advice," said a spokeswoman.

Availability

The WHO report raises the issue of whether emergency contraception should be made more readily available.

Currently, women need to go to their GP or family planning clinic for a prescription.

Toni Belfield, director of information at the Family Planning Association (FPA), said one option which could be considered was giving emergency contraception to women alongside ordinary contraception.

"At the moment, women have to jump through hoops and hurdles to get emergency contraception. It is ridiculous," she said.

She said doctors may not be accessible in the 12 hours after intercourse.

The FPA wants a new mechanism for delivering emergency contraception which it says could be timed with the licensing of a new progesterone only pill.

Ms Belfield said the pill had proved more effective in preventing pregnancy and had fewer side effects than the combined oestrogen and progesterone pill currently available in the UK.