A new morning after pill that could prevent pregnancy for nearly a week after unprotected sex has been hailed as an “exciting” step forward by the pro-choice lobby.

The drug, ulipristal acetate (UA), provides a contraception ‘window’ of up to five days compared with just three for the traditional emergency pill.

New research showed that it more than halved 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 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.

Women attending clinics in the UK, Republic of Ireland and the US seeking emergency contraception within five days of unprotected sex were randomly given one of the two pills. Among the total of 1,694 women, there were 15 pregnancies in the UA group and 22 in the levonorgestrel one.

Three out of 203 women who asked for emergency contraception between three and five days after sexual intercourse became pregnant, all of whom had taken levonorgestrel.

The earlier trial involved more than 1,500 women who were either given UA or levonorgestrel with a time limit of 72 hours. In this study there were seven pregnancies in the UA group and 13 in the levonorgestrel group. Combining the results of both trials showed that women taking UA were almost half as likely to get pregnant as those taking the traditional pill within five days of sex.

If emergency contraception was used within 24 hours of unprotected sex, UA reduced the pregnancy risk by almost two-thirds compared with levonorgestrel.

Ann Furedi, chief executive of the abortion charity the British Pregnancy Advisory Service (BPAS), said: “This new type of emergency contraceptive pill is exciting news for those of us working to help women avoid unintended pregnancy. It offers a longer time window for use than the traditional emergency contraception pill.

“We very much welcome research and development into contraception that can be used after sex. No method of contraception is 100% effective and couples are not always able to use them correctly, or at all.”