A new type of morning-after pill is more effective than the most widely used drug at preventing pregnancies after unprotected sex, claim experts.
The new study also claims it works longer by up to five days.
Levonorgestrel, the most widely used emergency contraceptive pill, is only effective if women take it within three days of having sex.
It is sold under various brand names, including Levonelle and Plan B, and is available in more than 140 countries, including the US, Canada and many countries in western Europe.
In nearly 50 of those countries, women can get it without a prescription.
International researchers compared the drug to the new product ulipristal acetate, sold as ellaOne in Europe only with a doctor's prescription. The drug is not legally on the market elsewhere.
Experts tracked nearly 1,700 women aged 16 to 36 who received emergency contraception within three to five days of having unprotected sex.
About half got Plan B, while the rest got ellaOne.
In the group that got Plan B, there were 22 pregnancies. In those that got ellaOne, there were 15.
The research was paid for by ellaOne's maker HRA Pharma, which helped design the study.
When the researchers pooled their results with a previous study comparing the two morning-after pills, they found women who took ellaOne within five days after sex almost halved their chances of becoming pregnant compared with women who took Plan B.
Women who took ellaOne had a 1.8pc chance of becoming pregnant, while women who took Plan B had a 2.6pc chance.
But health officials warned that this should not give women a false sense of security.
"The message has to be always that women should act as soon as possible," said Tony Kerridge, a spokesman for Marie Stopes International, a non-profit sexual health organisation in Britain not linked to the study.
"You may think you have a window of opportunity, but as soon as you can, go somewhere and get it sorted," Kerridge said.
Dr Anna Glasier, who led the Lancet study, said more safety data is needed before ellaOne could be recommended for over-the-counter use.