The contraceptive is available only on prescription
A recently licensed type of emergency contraception may offer women protection from pregnancy even when taken five days after sex.
Scottish researchers found that ulipristal acetate worked well after the three-day limit of the most commonly used drug, levonorgestrel.
At present ulipristal - unlike levonorgestrel - is only available with a prescription.
The British Pregnancy Advisory Service welcomed the study.
Emergency contraception uses hormones to either prevent the release of an egg by the ovary in the hours after sex, or stop it implanting into the the womb.
Levonorgestrel is available from pharmacies, either with a prescription, or sold directly to over-16s.
A study by specialists working for NHS Lothian tested the effectiveness of levonorgestrel and ulipristal (which was licensed for use last year) using a sample of more than 1,600 women from the UK, Ireland and the USA.
A total of 2.6% of the levonorgestrel group became pregnant despite taking the drug, compared with 1.8% in the ulipristal group.
In a much smaller group of women who received emergency contraception more than three days after sex, there were no pregnancies among women who had taken ulipristal compared with three pregnancies among those taking levonorgestrel.
The levels of side effects were roughly the same in both two drugs.
However, researchers said that the newer drug cannot be sold 'over-the-counter' at pharmacies because it did not yet have the established safety record of levonorgestrel.
Ann Furedi, chief executive of the British Pregnancy Advisory Service described the new type of drug as "exciting news".
She said: "It offers a longer time window for use than the traditional, emergency contraception pill.
"Different hormones are involved to the ones traditionally used in contraception, so it may be that these will prove to have other contraceptive uses in future.
"However, accessibility is key to the uptake of any time-sensitive medication and since this pill is not currently available over-the-counter and is significantly more expensive to buy than the traditional 'morning after pill', it may be that many women who could benefit from it are not able to access it."