A new "morning after" pill that can prevent pregnancy for nearly a week after unprotected sex has been hailed as an exciting step forward by pro-choice campaigners. The drug, ulipristal acetate (UA), provides a contraception "window" of up to five days, compared with just three for the traditional emergency pill.

Research has shown that it more than halves the risk of pregnancy compared with the 72-hour pill, levonorgestrel. However, although UA has been licensed in Europe since last May, it is not yet available over the counter in Britain and costs three times more than the alternative drug.

Doctors writing in The Lancet medical journal combined findings from their own trial with data from an earlier study. They found that if it was used within 24 hours of having unprotected sex, UA reduced the risk of pregnancy by almost two-thirds compared with levonorgestrel. The new drug could be made available from pharmacies and nurses if it meets safety requirements. Ann Furedi, the chief executive of the British Pregnancy Advisory Service, said: "For the last two decades, advances in contraception have been slow compared to other forms of medical innovation. This is the kind of research that modern society needs."