The British Pregnancy Advisory Service is offering free supplies of the morning-after pill to women who want to keep it at home over the Christmas period.

Guidelines from the Royal Pharmaceutical Society allow high-street chemists to provide women with emergency contraception before they need it, but some pharmacies will not dispense it and the service is not advertised. Few women know they can get the morning-after pill to keep at home.

The initiative by the BPAS is also aimed at girls and women who may not be able to pay the £26 the pill costs at a pharmacy. "Sex isn't always planned or prepared for," said Ann Furedi, its chief executive. "It's easy to get carried away, which is why we advise women to back up their birth control by keeping the morning-after pill at home. You don't wait until you get a headache to buy your pain relief - why wait until you've risked pregnancy to get the morning-after pill?"

The move will be controversial. Opponents of the morning-after pill fear it encourages a casual attitude towards sex, and may make young people in particular more likely to sleep around.

The BPAS says that closures of shops and GP surgeries over the Christmas season can cause major problems if a condom breaks or a woman forgets to take her pill and has unprotected sex. Although it can work up to 72 hours after sex, the morning-after pill is most effective the sooner it is taken. Its efficacy drops from 95% within 24 hours to 58% between 49 and 72 hours.

It is possible, the BPAS says, that women will find it hard to get hold of the pill soon enough over the Christmas period and would therefore be much better off having it at home just in case.

BPAS clinics will offer any women who want it a Christmas pack, which also includes condoms because the pill does not protect against sexually transmitted infections. Those who turn up will also be given advice on normal contraception, which is more effective at preventing pregnancy than the morning-after pill.